Your chance to earn this is in the first 15 minutes of gameplay. I did it as a Monk on normal difficulty, but I recommend story difficulty. Monk is the best class for this, as it's the only class that can make two attacks a turn with modifiers applying. A two-weapon fighting Fighter might be better if you only want to get this achievement, but two-weapon fighting is pretty weak overall. A Cleric using sacred flame might have a better chance to do damage.
Make sure you save Shadowheart. When you get to the bridge, you'll see three imps, a hellhound, and Zhalk fighting a mind flayer. Have your Githyanki fighter companion deal with the dog, and ignore the imps. Shadowheart should stay ten feet back and use Sacred Flame; it'll have a better hit chance than her mace. Move to Zhalk.
On normal, Zhalk has 150 hit points, and you have 15 turns before the ship crashes (and you die). However, After 8 turns, two level 4 devils spawn behind you, will take two turns to reach you, and then quickly kill you. So you really need to do this in ten turns. That means counting on the mind flayer to get lucky on scoring hits for you. Give it a few tries.