As you make your way through the ice realm, you will eventually face off with the primordial giant and the progenitor of the race of frost giants, Ymir. you cannot proceed with the story line until you defeat Ymir, but luckily it is an easy pattern.
First take out one of the smaller giants. He will drop a great polearm, which can be used to defeat Ymir... Dodge & weave until he smashes the ground with his club and then take whichever hero has the polearm and run up to that club before he lifts it again. This starts a quicktime animation sequence where you will be prompted to press several of the main buttons (X,Y,B,A) in a random sequence. Each button will advance you up Ymir's body, towards the back of his neck. At the end of the sequence, you will be prompted to repeatedly press the B button to jam the weapon into Ymir's neck, which will take his life down by 20%.
Rinse and repeat until you have taken him down.