Skul: The Hero Slayer

You don't need to fight the entire battle as Little Bone, just use him to land the final, killing blow on the final boss and you should get the achievement.

I got this achievement today. I dident land a single hit on the boss (with the little bone that is, main build was alchemist), my spirits did all the work. Just be the little bone when the boss runs out of HP.

To make this achievement more easy, go into settings and turn on rookie mode. Its still a though game but with the right build its doable.