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