Ratatoskr is a recurring room guardian who appears several times throughout the game.
Mausoleum of the Giants (A-5).
Immortal Battlefield (C-3)
- He jumps on the platforms in a W shape (literally). He moves toward one of the W points based on your position toward him. Just need to space yourself accordingly to manipulate where he jumps.
- Oh, also, make sure to stay on the ground level while doing this.
- Never need to jump onto any platforms; just move left and right to manipulate where he goes, and jump only to attack the head.
- You will be rewarded a Crystal Skull.
Then, he will be back at Icefire Treetop (G-2). This one was difficult... until I realized you can just stand in the center where the chest is and throw Spears at his head at an arc until he retreats.
This battle occurs in Eternal Prison - Gloom at (A-5). When he uses his laser attack, stand very close to his head and whip him as quickly as possible. Having the Vajra helps a lot here. He drops an Ankh Jewel when he retreats.
Hraesvelgr is the true form of Ratatoskr. You will need to use the Djed Pillar to chant the "Missing" (Mani) Mantra in all of the previous encounter rooms (with the exception of the first) to seal Ratatoskr. Also, you have to defeat Nidhogg if you haven't done that already. If you've done both, it will reveal its true form, Hraesvelgr, in Eternal Prison - Doom at (C-5).