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