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.
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.
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.