After you dispatch of the Shaman and his cronies you will run into another moose! Examine the moose corpse so you can get another part of the moose achievement.
You will then see a cave guarded by a troll. After you kill the troll you can enter the cave. Inside the cave there’s a fridge and inside the fridge is a Frost Staff. Personally I’d stick with the Staff of Life if you’ve managed to keep it until this point.
Head North and you will finally reach the Boss battle with the giant serpent named Jormungandr. The best way to kill this guy is with a bubble shield or rock walls that you put up. He will hit his head against them and do massive damage to himself. You can try shooting him with beams/projectiles but I think the bubble shield/rock wall strategy is the most optimal. An additional tip is he not charge you if you’re hiding behind a rock wall and are too far away. He’ll just spit acid on you. So you just need to walk near him and he’ll start charging again.