Thor

He raised the hammer and slammed it down on Skrymir's forehead.

[...]

He clambered up on to the giant's head once more. He positioned himself between the giant's eyebrows.
Thor spat into his hands. He adjusted his belt of strength. He raised Mjollnir over his head. And with all his might, he swung. He was certain that the hammer head sank into Skrymir's forehead.

[...]

This time he aimed for the giant's temple, and he hit Skrymir with all his strength. Never was there such a blow. Thor heard it echo from the mountaintops.

[...]

"Do you remember Skrymir?" he asked.
"The giant? of course."
"That was me. I used illusion to make myself so large and to change my appearance. The laces of my provision bags were tied with unbreakable iron wire and could be undone only by magic. When you hit me with your hammer, Thor, while I pretended to sleep, I knew that even the lightest of your blows would have meant my death, so I used my magic to take a mountain and put it invisibly between the hammer and my head. Look over there."
Far away was a mountain in the shape of a saddle, with valley's pluging into it: three square-shaped valleys, the last one going deepest of all.
"That was the moutain I used," said Utgardaloki. "Those valleys are your blows."