Darksiders

For the most part you will be working on this right from the very start of the game.

Before you forge the Armageddon blade, youll want to do this (as it replaces your Chaoseater and wont be able to work on this weapon).
NOTE : Supposedly Choaseater does not need to be upgraded according to the comment. Keep in mind, i wrote this guide as i was playing, and any edits that were in this guide were lost in the great TA crash/losing of data.
Please also note the Choaseater listed below is a recommendation not a MUST use.

First up -
Weapon enchanments.
Recommend
Chaoseater - Fury's Embrace
Scythe - Death's Blessings
Tremor Gauntlet - War's Glory

Second -Where to find
War's Glory - Legendary - improves chaos regen and weapon
After you fight the first large crystal/hungry spider youll come to a passage with 1 shadowflight blue orb and lots of abyssal chain orbs, jump into the shadow flight orb and pull back on the stick for a secret ledge and the Legendary weapon mod

Fury's Embrace - Legendary - reveals all items on maps
Enter black throne, head east, past the outside area.
When you get to the 2 button on opposite end of a chasm that produce a rock pieces brigge do the following.
Have void walker item ready, activate bridge, as your moving across, you need to look up and to the left and fire a shot at the void design. Once over the other side of the bridge, hit the button and make your way back to the void design. Use void, walk through void, press button and cross another collapseable rock bridge to recieve this Legendary weapon mod.

Blood Thirst - Leeches health when damaging enemies
Enter the twilight catherdral, cross the lava and go right through the door, drop down hole in this courtyard. Follow to end open door, turn left, go straight ahead, keep heading west. Take elevator down, in this room there are 2 lit sconces and 1 unlit, use crossblade to light the one to get your chest.

Strife's Offering - Legendary increase gun effectiveness
Ashlands, jump down the drill hole, follow until you come to the section with 12 plant teeth hanging from the ceiling. Go towards and use the shadowflight orb, then Abyssal chain across. After killing everything receive your reward.

Death's Blessing - Legendary -
Head north out of Scalding Gallows, youll come to the huge door that requires the Chrono puzzle to get past.
Place a void on the void design.
Activate the chronomancer the switch and make it past the big gate.
Once the gate crashed down look at it, place the other powered up void hole on it.
Activate the switch and run back to the chronomancer puzzle room.
Activate the switch, look through the void till you see the chest then jump through for the weapon enhancement.

Third - Why these enhancements,
Having all these slotted seem to stack, when using the weapon with these slotted i always gained health back, even though the weapon i killed with didnt have a leech ability.
The time it took me to level the last half bar with the Scythe using Death's Blessing, and the last half bar with the Tremor Gauntlet using War's Glory were.
Scythe+Death's = 1/2 bar = 15minutes
Tremour+War's = 1/2 bar = 15minutes

I did 1 1/2 levels on the Tremor gauntlet, and 3/4 a level on the Scythe and clocked up approximately 12000 souls.
This will help if you need extra souls to purchase the weapon abilities.

Fourth - Purchaseable items.
These are all the Chaoseater/Scythe/Tremor Gauntlet moves.
You do not have to purchase any the equipment(Crossblade/Mercy) upgrades.

Fifth - Best moves to use.
Sword - 180 orbital, (X pause X) or hit X,X, then wait a moment and repeat.
Scythe -
(Return Slash)
after you round them up, Hit Y,Y then wait a moment and repeat.
(Repear's Revenge)
Or the push foward and hold RB+Y (this will so a sliding swing attack)
Tremor Gauntlet,
(Earthshaker)
Stand still and press RB+Y, this will do a ground slam and pummel.

Sixth - Equipment
Abyssal Chain
Pull stragglers toward you to finish them off, but not a necessary piece of equipment.

Seveth - Location, Location, Location
Iron Canopy.
After defeating Silithia(or before it, its just easier to describe the location working backwards).
Leave the Silithia room and go back the way you came,(this is web door number 1). Work your way back and go through webdoor number 2..
Take a left and head down the hole, this next room has a load of spiders on the wall.
Round them all up and use your ability.

Now here is the trick to reseting this room.
After you have cleared out the room, climb back up the growth, dash forward 3 times, this will put you on a non-concrete floor. (This is the reset area - tested to see if this could be done any closer and it couldn't.)
Turn around and go back into the room all spiders respawned.

Final part
Once you have all the moves and the weapons upgraded, and have collected all the Armageddon blade pieces. Go get your Armageddon Blade recrafted once the cutscene is done, you will receive this achievement.

If you formed the Armageddon blade before you bought the last pieces from Vulgrim, the achievement should pop once you exit Vulgrim's store.

The description tells you what you need to do. You need to buy all the moves from Vulgrim and get all of your weapons to level 4. You don't have to kill enemies to get weapon xp, you just have to damage them and you will get a little bit. Killing them with just one weapon however will significantly increase xp earned. The best way of doing this is using the legendary enhancement called War's Glory. War's Glory significantly increases the damage done by weapons and gives you a huge boost in weapon xp.

To get War's Glory you need to be in Silitha's dungeon. After you get the Abyssal Chain go back to the alley way with a broken fire escape at the end of it. At the top of those stairs you should see a orange grapple point inside of the building in front of you, in there you will find the enhancement.

Make sure to equip it to any of your weapons to get the best out of it. After that on weaker enemies make sure to use your secondary weapons, the Scythe, and the Tremor Gauntlets. You should be able to get to at least level 2 using this method.

If you didn't do this the next best thing you can do to grind for weapon xp is go to Silitha's dungeon again and fight the spiders. The spiders constantly respawn as you move throughout the area and there is almost always a group of at least six in any given area. Loop around the dungeon for constant waves of enemies to kill.

As a final note, the Armageddon Blade starts out automatically at level 4, so you do not have to worry about leveling it.

EDIT: You don't have to buy the imbuements for your equipment such as the Crossblade or Reaper, just the moves for the Chaosblade, Tremor Gauntlets, and Scythe

Darksiders weapon and EXP Leveling Help video, should help for leveling up weapons and EXP in Iron Canopy. Video by 0NinjaViking0, no credits for me.

Just want to add that you need to buy all the upgrades (to lvl 3 I think) of all moves to the three main weapons from Vulgrim before this unlocks.

To assist / clarify even further for people looking to grind. The best spot I've found so far is also in the Silitha dungeon. On your way to find the abyssal chain you're going to move through a large room with groups of spiders all along the wall. After you clear this room you'll head into a small hallway, turn left, climb a small wall of demon growth then head outside to a larger area. You'll know you're in the right spot if the boos door is ahead and on your right.

From the top of the demon growth, dash forward three times (you should have reached a point where the ground changes to what looks like a tiny section of hardwood). Turn around and head back into the previous room. All the spiders have respawned. Rinse and repeat.

While this room will always respawn like this, I found it's actually easier (albeit slower) to grind the room now BEFORE you get the abyssal chain. The reason being is that after you get the chain, one of the spider mini bosses is located in this room. Once you defeat it, the spiders in this room turn more aggressive and attack you on site, and there are fewer mobs of them. Granted you won't have the War's Glory enhancement, but if you're not very good with the scythe or the gauntlet it may be worth your time to ignore getting it til after you've put in some grind time.

A good side note is these spiders are very easy to melee should you need health. Meleeing the whole room should net you a full health bar each time.