@Lalliman: Skin with the hardness of diamond wouldn't have the brittle problem, assuming it still somehow functioned like skin.

@Vigild: Pretty much what Lalliman said, but diamond that isn't brittle is quite a bit more durable than anything else involved here, and you currently have no limits on any of his superhuman abilities. He could give himself diamond hard skin while simultaneously sticking to a wall and incinerating an enemy two kilometers away, at the same time he's smacking another enemy with a piece of straw that carries the kinetic force of a semi truck. Put simply, it's not what he can do that's the issue, but rather how much/often he can do it and the fact that it has no draining effects on him. All of the abilities (including the diamond skin) would be fine if they had sufficient limits placed on them.