Ratatoskr

Ratatoskr is a recurring room guardian who appears several times throughout the game.

First Encounter
Mausoleum of the Giants (A-5).

Second Encounter
You will run into Ratatoskr after whipping the statue in Roots of Yggdrasil (B-1) tied to the Ankh puzzle.

Third Encounter
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.

Fourth Encounter
After the encounter in Immortal Battlefield (C-3), you can run into Ratatoskr in Icefire Treetop at (C-1): He's holding the Ankh Jewel that was once in this room.

Fifth Encounter
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.

Sixth Encounter
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
Hraesvelgr is the true form of Ratatoskr. You will need to use the Djed Pillar to chant the "Missing" (Máni) 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).