Dungeon Hunter 5

This story related achievement is acquired from finishing mission 17. This mission ends with another boss that is quite strong. It's better to stay away from it until it uses one of its many attacks, and then move in and get a hit or two while it recovers. It will create a shield and summon minions multiple times during the fight. If you have a good crossbow that would probably be even better. I would suggest having either enough gems to resurrect or some healing potions before doing this mission.