"Dolly" -Vice! "Bumping hammer" -Standard claw hammer, will be fine for banging out a dent in an aluminium chassis. You will only stretch it if your "dolly" is right behind the hammer face. Heres a tip, hold the "dolly" on a spot next to the dent, and then hit it the dent down. This is typically called an "off dolly" hit and as long as you dont go crazy will not stretch the material. If you can get away with just bending it in the vice or similar I would do that first as you have more control...