This achievement can be unlocked very early in the game (Level 5 onwards) and I actually got this achievement without even focusing on it. The easiest class to get this with is Rogue Archer. Get a bow with as much DPS as possible then get the ability "Full Draw". With this ability you can hit well over 1000% damage as long as the enemy is at full health. This does not need to be done by your character, as long as it is done by your party.
Just keep playing the game and levelling up and getting as much DPS and damage increase abilities as possible.. You WILL get this.