To unlock this achievement, on the third and final island and you have completed all tasks and the assault on Hel's Fortress is availble, call the attack. This isn't a very easy task, but you must survive the whole battle without being summoned once more by your shaman. If you are killed, quickly press start and you quit out, and restart again, the battle option will still be availble, when you finally reach Hel, undefeated, it is ok to die. There will be no penalty and the achievement can still be unlocked after she is defeated.