Recruiting giants was not going to be an easy task. Even with Thor's severed head. Well, not all of the giants were tough to influence. Arnold was pretty sure that with Mimir's support, he would be able to fool them into taking up arms for him.
But there was a problem... the fire giants. Even though the fire giants were enemies of frost giants, when Thor killed the latter, the fire giants were not pleased with the end result. In their mind, Thor had taken away their enemies from him and believed that Thor or any other god for that matter did not have any right to interfere in their issues.
This was when the fire giants were losing the battle horribly. If it hadn't been for Thor's anger, they would have been the ones to get... extinguished. It was more like a loser's barking, but as time went on the fire giants actually started believing the crap their leader, Surtr was spewing out.
