Can be done as early as lv.5 ( EDIT: Level 4 as a Human with the bonus skill point. ) starting as a Rogue Archer, The ability you want to use is called 'Full Draw' and it's effect is that if you hit an enemy at full health it does up to 1600% damage with your bow. using the 'Bow of the Dragon' (which has 62DPS) I was able to hit upwards of 1400.
This achievement also appears to be buggy as it didn't unlock until maybe the 7th time I hit 1000+, at which time I hit a 1002, so If it doesn't unlock the first time, just keep trying until it does, you'll get it eventually.
EDIT: The achievement is NOT specific to your character, and can be unlocked with a party member, so if you're not playing a rogue you can still unlock this achievement with any of the 3 rogue companions in the game.