The rain is unexpected. Peter suddenly finds himself having to deal with a very slick battlefield. He can outrun an explosion. And he can run on water. But running on a muddy field is a much different experience.

Include that with the fact that he has to dodge the many lightning strikes... and Peter finds himself slipping and sliding into the mud. And hit by another bolt.

"Okay! You got me! Twice!" He holds out his hands. Covered in mud.