Thor: God of Thunder

On Vanaheim, you'll get on a massive barge with a Mire Giant pulling you along. At the end, you'll have to destroy 4 Mortars which are bombing your pet Giant, you need to destroy these whilst leaving your Mire with 40% or above health. You destroy them by beating up the enemies around them and using Rage. Below is the tactics i used for my Valhallen Difficulty run and i finished with exactly 40% health.



-- Checkpoint --
At the start theres a Purple Rune that will fill all your Health and Mana. This is also the checkpoint should you fail or wish to restart,

-- Mortar 1 --
Head up for a cutscene and rush forward to the first Mortar. Don't use Magic. Instead use XXX,Y (use the hold Y version if you have it). You should finish these up quick, they have low health but can do some damage. When your rage is full quickly Tap R1 and be ready to run to the next Mortar. Your Mire should be on 90% health.

-- Mortar 2 --
Head over the log to the 2nd Mortar, on the way your Mire will be hit with a Mortar. Unavoidable. Jump up and click in your Right Stick and throw your hammer repeatadly at the Clan Troll. Make him your main priority. Then finish off the rest like before, don't worry about magic this time, but save some for the trip to 3rd Mortar.

-- Mortar 3 --
On the way to the 3rd Mortar you will need to fight 4 creatures with Cannons. These hurt so use Wind Magic to blow them off the edge. Jump up to the 3rd Mortar and on your right is an Odinforce Pickup. Grab it and use your powered up Lightning attack (Hold Y). Make sure you kill all 3 of the Trolls whilst you still have unlimited Mana. If your Mire is on 50% health, your good, the next part he'll only lose 10%.

-- Mortar 4 --
When you jump to the 4th Mortar you should have full magic still, from the pickup. Use powered Lightning (Hold Y) to kill the Troll and finish off the rest with Lightning (Tap Y) and hammer throws.

-- Finish --
The achievement pops as soon as the last Mortar is detroyed. It's often best to go through once slow, so you know the enemy and directions to go. Then when your ready to Rage the final Mortar. Instead Quit and Continue, you'll be back at the Purple Rune.

This was my method on my Valhallen run through, so it should work better on a lower difficulty.
Lower difficulties it is easier to use powered hammer throw (Right Stick + RT) on the 3rd section

You have to race through destroying the turrets that are bombarding the barge before they destroy it. You can only break them by using "Power Surge" (RB) which is available when you fill your fury meter by quickly defeating foes in succession.

For the first stage just kill the small enemies quickly, try using AOE attacks. Then for three other turrets that have trolls just spam "Earthquake" which deals massive damage and breaks armor, make sure thunder abilities are selected and hold and then release (Y).