I answered a similar question recently; you can find it here. Speaking only for Ancient Greece, the answer is that even though we get descriptions of piles of bodies in the aftermath of battles, there is no evidence that they were enough of an obstacle to require infantry to change its approach. This is probably because the number of casualties in the actual clash of the lines was much lower than Hollywood would have you believe, and most of the losses suffered were killed in the pursuit when one side had broken.

By contrast, there are several examples of battles that were decided by sudden thunder and heavy rain.