I like to fire a few Mines onto the Manta before I jump in; usually 2 or 3, because any more than that are hard to place quickly. (They will stick if you place them correctly). Then, if a player on foot dodges me when I try to Pancake them, the spider mines attack! :-D
Always keep firing when flying front on towards an enemy to destroy incoming avrils.
|rH|Nietzsche
Mantas are great for the final assault against an enemy base, as they can move quickly and jump over the walls. Note that you can easily dodge avrils in a manta with well timed jumping and slamming. When running people over going down a hill, be sure to slam - this ensures that you won't go over anyone's head without killing them.