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