This is an easy achievement. About midway during the game (assuming you aren't just putting your upgrades on your ranged weapons) you have to defend two dwarves while they activate a weapon. There are two mounted crossbows nearby, go on them and start shooting at the Trolls. This should do between 3000 and 4000 damage, easily getting your achievement. If you don't do this, you can always just pour your points into leveling up your ranged weapons, and use your power shot on an enemy while they are in a critical state.