Heavy Hitter Achievement
Main character inflicted 250 damage with a single hit
As a Warrior and have a high strength and willpower. Get a two-handed weapon with runes on it to increase the damage you inflict. Have your party mates cast buffing spells on you. Use the Berserker skill “Final Blow”. Make sure you stamina bar is full as well.
As a Rogue, max out your Archery talents, especially the Pinning Shot line. Get to Arrow of Slaying as soon as you can. Use that talent in a fight and you'll easily do over 500 damage.
As a Mage, use Walking Virulent Bomb or Mana Clash on another Mage. Combining Spell Wisp, Spell Might, a high magic attribute, and two items that increases damage works as well (thanks to AstralBlades for that tip).
This needs to be done against one, single enemy. There have been quite a few times where I've damaged multiple opponents with my Mage or Rogue with damage well over 250. Remember, it's a single hit, not a single attack. You also need to do this with your main character. No switching to Shale. It won't count.