Here is the full list of all 18 Dungeon Defenders II achievements.
On four heroes, reach level 25.
Reach level 25 on a hero.
Activate 100 environmental traps.
Reach level 15 on a hero.
Deal 5,000,000 damage with Cannonball Towers.
Reach level 5 on a hero.
Deal 5,000,000 damage with Flamethrower Towers.
Deal 5,000,000 damage with Explosive Traps.
Deal 5,000,000 damage with Lightning Strikes Auras.
Defeat 100 Kobolds before they light their fuse.
Defeat 50 Witherbeasts before they finish burrowing.
Defeat 1,000 special enemies.