Ascend Hand of Kul

To access the upgrade menu:
Hit the back button -> press RB until you get to the spells tab -> highlight the spell you want to upgrade -> press A and select upgrade. You will then be able to select different slots on a rotating wheel. You must buy all these slots for one spell.

The starter spells for each god are:
Dark - Fireball, a damage dealing projectile that may ignite your target
Light - Ionized Bolt, damage target and stuns briefly
Void - Seekers, multiple damage dealing projectiles

You can have all 3 spells at once if you choose to switch to a different god when you ascend, and then purchase the starter spell for 10,000 souls OR unlock it via progressing in the main storyline. (Acquiring one spell from each god will net you the 'Carving A Path' achievement.)

Of these 3, I find Ionized Bolt to be very useful as it stuns the larger bosses. Seekers is good for damage. I rarely bother with Fireball.
Judging by the first few upgrades you can buy, I want to say Fireball is the cheapest, followed by Ionized Bolt, then Seekers. There are 12 upgrades for each spell, and the price of the upgrades can be up to 15,000 souls. Luckily, your spells and their upgrades will automatically legacy when you ascend, so it's worth investing in.

Tips For Collecting Souls

PICKING YOUR GOD

What god you pick at the beginning affects what starter spell you have. I feel that the most useful is the Light God's spell, Ionized Bolt, because it can stun the larger enemies for a few precious moments and allow you to land a few blows. Getting the charge upgrade makes it even more useful. Seekers (Void God) and Fireball (Dark God) are mostly for doing damage.
You can change the god you are aligned to every time you ascend. Your God determines the gear you can buy at the store, and what special dungeons you can access.

ACQUIRING DECENT EQUIPMENT

Don't buy from the menu store very often. Items there are very expensive. If you find that an enemy is shaking off your blows, then spend your souls on a better weapon. Better armor can be found in chests and is where you'll be getting most of your gear. Many chests can be found in dungeons. Once you reach the Battlements early on, there are 5 dungeons in that one area. Chests reset every day and every time you ascend.
YOU WILL NOT BE WEARING YOUR BEST GEAR CONSTANTLY.
Keep every item you find. You can always sell it later if you really don't need it. Use your weaker gear until it breaks. Then switch to backup gear if you have any, and sell the broken gear. If you don't have any backup gauntlets or boots, it okay to run around without them, but always try to have armor. Keep in mind that if you switch to your best gear, and you don't have any backup gear, you cannot unequip it. Only use your best equipment against tough enemies, and repair after every fight. If you wait to repair until the gear is broken, it can cost thousands of souls. If you found really good equipment that is nearly broken, or accidentally let you best gear break, it might be more efficient to upgrade and repair. Level II upgrade is usually only a little bit more expensive.

ASCENDING

When you ascend, you have the choice to "Legacy" an item, making it available after you ascend and every time you ascend later on. You do not need to legacy an item if it is already marked as a "legacy item" Needless to say, legacy your best item. You get to legacy one item for free. You can purchase more pillars that allows you to legacy more items. The first cost 1K, the second 2K, the third 4k, and it doubles every time. I would only buy one or two pillars if you have gear you can't afford to lose. The pillars do not carry over to the next time you ascend. Any items you don't legacy, including runes, curses, and blessings, will disappear. Sell these items before you ascend. Money will carry over, as well as spells. I would advise not selling legacy items unless they become completely useless. You will keep it every time you ascend, meaning a free backup item. When you ascend, you get to pick gear to wear, like when you first started the game. Always pick the free gear. You do not need to equip the legacy items for them to carry over into your inventory.

FARMING SOULS

After you've converted shrines and altars, you may get a notification pop-up that you are being invaded by another person's ascended champion. There will be a timer for 5 minutes. When the time runs out, they will have converted your shrine or altar, but will remain in the area. Killing them will net you 400-500 souls, and they will respawn if you do not convert the shrine or altar back to your god. Fighting them DOES NOT deteriorate you weapon or armor! However, if they are at an altar, the human enemies will be on their side and will attack you and damage your gear. So, ideally, a shrine located near the beginning of an area would be a good place to farm. I would not recommend fighting a champion a few levels ahead of you, but those a few levels below you will be easy to defeat and just as rewarding. Tactics vary with champions. Some use a lot of magic and some don't. All champions seem to block a lot, so use heavy attacks and block when necessary.

TIPS FOR COMMON ENEMIES

The less you get hit, the less you'll have to repair your gear. RT+X is your friend. Many of the tougher enemies can be avoided by simply running past them. Even if they spawn near a chest (like ogres at the end of a dungeon), you can grab the loot and run for it. Killing ogres rewards you with quite a few souls, but not worth it until you learn to avoid most of their attacks. There is no penalty for dying, but taking many hits will damage your gear.
Also, if you see an enemy with an A button indicator on it's head, that means you can press A for an instant kill that replenishes some health and gives you extra XP.

Trolls
-These guys are everywhere and attack in packs. You will be getting most of your souls from them. Use the quick X attack on them. The Y attack is too slow and you likely get hit if you try to use it. Try not to get surrounded, and if you do, make sure to hit the ones at your back to stagger them so they can't attack for a few moments. Your goal against these enemies is to build up your hit combo so you make more souls.

Bats
-Same strategy as with Trolls, except they have a screech attack with a bit of range on it that may ruin your combos.

Witch Doctors
-These creatures float, hit with ranged magic, teleport, and are very annoying. Because of their ranged attack and potential to ruin combos, target them first. Either relentlessly dash at them (RT+X) and try to get them against a wall so they can't teleport away, or attack them with magic. They explode when they die, so dash away if you killed them with melee.

Shield Trolls
-Target these enemies last in a fight. They are slow moving, so kill any trolls around them and keep moving away. Use your heavy Y attack to break through their shield, then follow up with a X+Y combo. They tend to block magic, but sometimes it gets through. Watch out for when they raise their shield, they're preparing for a heavy attack and will break through your blocks.

Berzerker Trolls
-These are larger than your average troll, red and black, and more aggressive. They can cast frenzy on themselves and any other trolls around. I tend to avoid melee with them and cast spells, but if you must melee, remember to block.

Ogres
-These guys are huge, and wield a giant weapon. Take out any enemies that might have spawned with it, staying away from him. When he's the last left, stun him with Ionized Bolt (or any magic that can stun briefly) and throw in 2-3 heavy hits before he recovers, then RT+X away. If you have several spells, keep hitting him with them. Unless you're about to stun and attack him, keep your distance. He hits hard.

Ogre Brutes
-The biggest common enemy you'll face. They're usually at the end of dungeons, or in areas you revisit. The tactics for fighting these are much the same for fighting regular ogres. If you take too much damage, retreat until the Ogre Brute loses interest, wait for your health to come back, then engage him again. I've seen these guys pick up and eat humans to regain health, so keep that in mind if you're near a village.

If there is any misinformation in this guide or any improvements I can make, please comment and let me know.

To access the upgrade menu:
Hit the back button -> press RB until you get to the spells tab -> highlight the spell you want to upgrade -> press A and select upgrade. You will then be able to select different slots on a rotating wheel. You must buy all these slots for one spell.

The starter spells for each god are:
Dark - Fireball, a damage dealing projectile that may ignite your target
Light - Ionized Bolt, damage target and stuns briefly
Void - Seekers, multiple damage dealing projectiles

You can have all 3 spells at once if you choose to switch to a different god when you ascend, and then purchase the starter spell for 10,000 souls OR unlock it via progressing in the main storyline. (Acquiring one spell from each god will net you the 'Carving A Path' achievement.)

Of these 3, I find Ionized Bolt to be very useful as it stuns the larger bosses. Seekers is good for damage. I rarely bother with Fireball.
Judging by the first few upgrades you can buy, I want to say Fireball is the cheapest, followed by Ionized Bolt, then Seekers. There are 12 upgrades for each spell, and the price of the upgrades can be up to 15,000 souls. Luckily, your spells and their upgrades will automatically legacy when you ascend, so it's worth investing in.

Tips For Collecting Souls

PICKING YOUR GOD

What god you pick at the beginning affects what starter spell you have. I feel that the most useful is the Light God's spell, Ionized Bolt, because it can stun the larger enemies for a few precious moments and allow you to land a few blows. Getting the charge upgrade makes it even more useful. Seekers (Void God) and Fireball (Dark God) are mostly for doing damage.
You can change the god you are aligned to every time you ascend. Your God determines the gear you can buy at the store, and what special dungeons you can access.

ACQUIRING DECENT EQUIPMENT

Don't buy from the menu store very often. Items there are very expensive. If you find that an enemy is shaking off your blows, then spend your souls on a better weapon. Better armor can be found in chests and is where you'll be getting most of your gear. Many chests can be found in dungeons. Once you reach the Battlements early on, there are 5 dungeons in that one area. Chests reset every day and every time you ascend.
YOU WILL NOT BE WEARING YOUR BEST GEAR CONSTANTLY.
Keep every item you find. You can always sell it later if you really don't need it. Use your weaker gear until it breaks. Then switch to backup gear if you have any, and sell the broken gear. If you don't have any backup gauntlets or boots, it okay to run around without them, but always try to have armor. Keep in mind that if you switch to your best gear, and you don't have any backup gear, you cannot unequip it. Only use your best equipment against tough enemies, and repair after every fight. If you wait to repair until the gear is broken, it can cost thousands of souls. If you found really good equipment that is nearly broken, or accidentally let you best gear break, it might be more efficient to upgrade and repair. Level II upgrade is usually only a little bit more expensive.

ASCENDING

When you ascend, you have the choice to "Legacy" an item, making it available after you ascend and every time you ascend later on. You do not need to legacy an item if it is already marked as a "legacy item" Needless to say, legacy your best item. You get to legacy one item for free. You can purchase more pillars that allows you to legacy more items. The first cost 1K, the second 2K, the third 4k, and it doubles every time. I would only buy one or two pillars if you have gear you can't afford to lose. The pillars do not carry over to the next time you ascend. Any items you don't legacy, including runes, curses, and blessings, will disappear. Sell these items before you ascend. Money will carry over, as well as spells. I would advise not selling legacy items unless they become completely useless. You will keep it every time you ascend, meaning a free backup item. When you ascend, you get to pick gear to wear, like when you first started the game. Always pick the free gear. You do not need to equip the legacy items for them to carry over into your inventory.

FARMING SOULS

After you've converted shrines and altars, you may get a notification pop-up that you are being invaded by another person's ascended champion. There will be a timer for 5 minutes. When the time runs out, they will have converted your shrine or altar, but will remain in the area. Killing them will net you 400-500 souls, and they will respawn if you do not convert the shrine or altar back to your god. Fighting them DOES NOT deteriorate you weapon or armor! However, if they are at an altar, the human enemies will be on their side and will attack you and damage your gear. So, ideally, a shrine located near the beginning of an area would be a good place to farm. I would not recommend fighting a champion a few levels ahead of you, but those a few levels below you will be easy to defeat and just as rewarding. Tactics vary with champions. Some use a lot of magic and some don't. All champions seem to block a lot, so use heavy attacks and block when necessary.

TIPS FOR COMMON ENEMIES

The less you get hit, the less you'll have to repair your gear. RT+X is your friend. Many of the tougher enemies can be avoided by simply running past them. Even if they spawn near a chest (like ogres at the end of a dungeon), you can grab the loot and run for it. Killing ogres rewards you with quite a few souls, but not worth it until you learn to avoid most of their attacks. There is no penalty for dying, but taking many hits will damage your gear.
Also, if you see an enemy with an A button indicator on it's head, that means you can press A for an instant kill that replenishes some health and gives you extra XP.

Trolls
-These guys are everywhere and attack in packs. You will be getting most of your souls from them. Use the quick X attack on them. The Y attack is too slow and you likely get hit if you try to use it. Try not to get surrounded, and if you do, make sure to hit the ones at your back to stagger them so they can't attack for a few moments. Your goal against these enemies is to build up your hit combo so you make more souls.

Bats
-Same strategy as with Trolls, except they have a screech attack with a bit of range on it that may ruin your combos.

Witch Doctors
-These creatures float, hit with ranged magic, teleport, and are very annoying. Because of their ranged attack and potential to ruin combos, target them first. Either relentlessly dash at them (RT+X) and try to get them against a wall so they can't teleport away, or attack them with magic. They explode when they die, so dash away if you killed them with melee.

Shield Trolls
-Target these enemies last in a fight. They are slow moving, so kill any trolls around them and keep moving away. Use your heavy Y attack to break through their shield, then follow up with a X+Y combo. They tend to block magic, but sometimes it gets through. Watch out for when they raise their shield, they're preparing for a heavy attack and will break through your blocks.

Berzerker Trolls
-These are larger than your average troll, red and black, and more aggressive. They can cast frenzy on themselves and any other trolls around. I tend to avoid melee with them and cast spells, but if you must melee, remember to block.

Ogres
-These guys are huge, and wield a giant weapon. Take out any enemies that might have spawned with it, staying away from him. When he's the last left, stun him with Ionized Bolt (or any magic that can stun briefly) and throw in 2-3 heavy hits before he recovers, then RT+X away. If you have several spells, keep hitting him with them. Unless you're about to stun and attack him, keep your distance. He hits hard.

Ogre Brutes
-The biggest common enemy you'll face. They're usually at the end of dungeons, or in areas you revisit. The tactics for fighting these are much the same for fighting regular ogres. If you take too much damage, retreat until the Ogre Brute loses interest, wait for your health to come back, then engage him again. I've seen these guys pick up and eat humans to regain health, so keep that in mind if you're near a village.

If there is any misinformation in this guide or any improvements I can make, please comment and let me know.