This unmissable achievement is for completing the semi-tutorial prologue to the game. You're given three basic tasks:
1. Defeat a certain number of orcs without the hero dying. This gives you a chance to learn how to use various abilities.
2. Defeat a pair of ogres and a pair of catapults with your two heroes. This gives you a chance to try more skills, learn how to switch back and forth between heroes, and do a little more pause-and-think-your-next move strategy.
3. Defend the king while he closes the gate. Back to one hero, just don't let the king die before he's done.
After this there will be a cutscene and the achievement will unlock.