Valhallan difficulty is all about the Buzzsaw Throw (LT then RT) and the clicked in Hammer Throw (click in RS and then RT). The Buzzsaw throw will temporarily stun certain enemies so you can get a breather, click in and throw some hammers, or get some melee attacks in. Buy this power ASAP. Mjolnir Blast (best hammer skill) and Odinforce Regeneration (best general skill) definitely help.
I relied heavily on the Buzzsaw Throw during the boss fights, especially Ulik, Surtur, and Mangog. The clicked in Hammer Throw does a lot more damage than the regular Hammer Throw and allows you to keep a distance. Unfortunately it requires mana but the Buzzsaw Throw does not.
Once I figured out how to use the Buzzsaw to stun regularly, the bosses became a lot easier.
As for Mangog, this is what I did:
Stage 1:
- Circle around him constantly, never be standing in front of him.
- Buzzsaw him and if you stun him either click in Hammer Throw or jump up and melee him. I would click in Hammer Throw until I had two blue boxes left on the right of the big bar.
- For every quick time event do X, X, Y. For some reason X, X, X takes life away.
Stage 2:
- Try to dodge as much as possible during the first barrage of fireballs. They come in really fast and multiple at a time. They are hard to reflect and if you get hit by many of them you are going to get stunned in place and lose a lot of health. If you can reflect some, do it, but otherwise it's easier to reflect the final, big, slow ones.
- When he comes out for the quick time event, just hammer X all three times. You'll probably need the health.
Stage 3:
- Stage 3 is very similar to Stage 1 but now if you get too far away he'll throw fireballs.
- Just stay close to him, constantly circle him, Buzzsaw stun him, and then click in and throw hammers.
- As soon as he jumps, run and dash towards him. Then when he start swirling fire, click in Hammer Throw him in the head. If you hit him in the head it'll automatically max your Rage bar.
- As soon as your Rage meter is full hit RB and follow him to Loki. I found that X, X, Y was good for the quick time event. But X, X, X works too. I couldn't really figure out which one was better.
- If you can get your Rage bar full before he jumps then he might jump immediately after the quick time event giving you another full Rage bar.