This achievement unlock when you reach level 30 with Kruber.
A quick SOLO
leveling method is speed running through levels at Legend difficulty to get as far as possible, even if you die. The specific level that I feel is fastest is Empire in Flames
Running and pushing through enemies, getting to the first two scripted Chaos Warriors at the entrance to the city will get you around 200 XP. If you get even further and reach Elba Durbein’s Cellar
, you’ll earn around 450 XP. With loading times, your average run will be around 4 minutes, making this one of the fastest leveling methods in the game. Equipping movement speed items and utilizing characters movement abilities can speed it up.A specific strategy for Kruber is to use his third sub class, the Foot Knight. His ultimate move allows him to rush forward.Edit: Another great strategy with this map is this video here. If you have a Battle Wizard in your group, you can glitch outside of the map with her teleport ability. This can be done in roughly 5 minutes, and gets you 800 xp!
In terms of getting loot boxes it’s impractical, but if you just want the character achievements, or don’t vibe with a specific character, this will be the fastest way to get them to 30. Good luck, and let me know in the comments if I’m not clear enough on this method.