Sacred 3

You need to reach level 49 to be able to purchase the last upgrade in the skill tree found under "execution". There is no need to level up weapons and armor by playing the main missions for the achievement to pop.
My equipment was between level 1-3. Do this for all four original non DLC characters to unlock the achievement.

Found this video. All credit goes to youtube user deathmule.

Warning! As stated in the comments the achievement won't unlock if you maxed the Malakhim before the other characters. Keep that in mind.

Edit:
Just take this Video as an idea to know what you're doing. I'm giving advice now on what I think is the most efficient way to leveling up and grinding gold.

You have to do the first mission as a tutorial to have access to the multiplayer mode. After that either have a friend with a high level character beat the final mission (Ashgar) for you or join someone's game that is playing this mission and hope that he/she doesn't kick you.

After this you have access to the deity difficulty where you're gonna spend most of the time grinding.
I played some random missions on legend difficulty until I hit level 6 and then went straight for Icecreek Sanctuary.

Edit:
capicawl and Coggs25 stated in the comments that the first 3 difficulties give the same xp now so there is no need to play on legend diffculty anymore.
Deity still gives more xp so you should switch to this setting after hitting level 16-18 to get the best outcome.
Capicawl also mentioned that playing coop every player gets the gold that other players collect.

The level starts out with an ice golem. Kill him get lots of xp, then quit and restart. As soon as you unlock the 200% damage reflect upgrade for your block skill use it!
Makes things a lot faster.

Do this until you are level 16-18 depending on your personal skill and then switch to deity diffictulty.
Keep repeating the process until you are level 50.
Although I stated that level 49 is sufficient to be able to buy everything, I prefer having my characters at level 50 since you get a little more gold.


For the gold grinding I prefer the sidemission "Bravelake Flat".
Make sure you use the bombs that are lying around as much as possible since a direct explosion instantly kills any enemy.
Alone this mission takes me a good minute to complete but it can be done quicker with more people with clever usage of the bombs.
A successful run nets you ~2000 gold; slightly more with more players.
You need around 250.000 gold to max out one character so this is gonna take some time.

CAUTION!!! DO NOT MAX OUT THE MALAKHIM BEFORE ANY OF THE FOUR NON DLC CHARACTERS. THIS VOIDS THE ACHIEVEMENT.
He can be level 50 and have everything besides one skill/weapon/combat art maxed but NOT EVERYTHING.
I had to learn it the hard way.

I hope this solution helps some of you guys. If you vote negatively please tell me why so I can improve it.

Introduction
To get this achievement you have to get approximately 260.000 gold per character and buy all the upgrades from all the 4 main characters. The last upgrade unlocks at level 49. Nothing carries over from character to character so it's all done in parallel

Summary:
1) With a new character get to level 50 (read my guide on how to do that easily)
2) Get the Health Totem, Health Potion and optionally Energy Totem items and their upgrades
3) Get Deity difficulty unlocked by playing the last level and beating Zane on any difficulty
4) Repeat the mission Bravelake Flat or Gloom Vale

More detailed:
1) When you start out with a character you will have to beat the first level. There is no new game+ system so nothing transfers over. Luckily through an exploit you can become instant level 50. More on that you can read in my guide:
Solution for Veteran In Sacred 3
If you don't have a friend for the level up process you can choose to do the grindy part yourself. If you're going for the grind, start at level 40, you'll still get XP while gold grinding
2) Play the missions:
Halios
Skaldera
Agari Glen
Hermit's Path

Oakshade
Sunken Ramparts
Valam Lake
Bravelake Flat

And optionally:
Lost Catacombs
Fendrel Woods
Urkus Swamp
Goldpeak Mine

3)Now you're going to unlock Deity difficulty. You can either join someone playing the last mission Azgarth or you're going to unlock it yourself by playing all Main Missions (preferbly on champion difficulty)
4) With those items and their upgrades you should have some gold in your pockets. Except for the missions in bold it is wise to repeat the mentioned missions on Deity difficulty so you upgrade them further.
Either way now the grinding begins and you should keep repeating Bravelake Flat and / or Gloom Vale on Deity difficulty to keep getting around 2000 gold. This number is not fixed and you can sometimes get 3000 or merely 1600. Either way you will have to keep repeating those missions until you have enough gold. Make sure to use the 3rd upgrade for the Block so you repel x2 the damage done to you. Don't refill your potions etc, the auto-refill upgrades should keep you well enough stocked every time.

This process should take about 3,5-4,5h per character. If you still have to play through Deity difficulty on the campaign, do that first, that saves doing an entire character.
Good luck!

This is a warning post.

I wasn't far away from getting this achievement, lvl 50, lvl 50, lvl 46, lvl 39.
I completed the final boss on deity difficulty for a third time, my Armour upgraded to lvl 6(Glitched) which then froze my game. This then saved onto cloud, corrupting the file. Every time I tried to start the game, it would freeze. I had to delete the cloud save just to play the game. resulting in losing all progress.
All 4 characters gone, time to start again.

Only complete the final mission boss TWICE.
Once on legend difficulty for the achievement.

Then again on deity difficulty for that achievement.

This should get you max armour(lvl 5).
DO NOT complete the final mission boss again after this, you can play the final mission again, just quit out before you reach the boss.
This will save you from possibly glitching the game and corrupting your save.

First thing your gonna need is access to deity mode. Next get the character your trying to max out to lvl 26 or 27. Now put on block with the 200% damage reflect and start repeatedly doing the last stage asgar.

Reason: because you fight 4 bosses plus a a lot of grunts that give out lots of gold and experience. I tried the above methods but found them to be somewhat slow.

I say 26 or 27 because I was getting murdered really fast at levels lower then that.

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