Alright Snap, use a Faint Attack to rush forward and whack Mimic before he can lift the rocks, surely that will take time. Then as your right beside he might hit himself along with you. Next, make sure right up beside Mimic while he's using Shadow Ball it will explode right there and hopefully he'll be caught in the blast as well. While your there(but after the blast), Crunch him. Lastly, counter his Secret Power with a Flash Cannon.

Faint Attack/Crunch/Flash Cannon