I'm at work right now, and am going by memory...
In the arena setup options, on the weapons (I think that's the one) tab, only set the top three items to something less than 1. Those affect the auto ack (puffy and non-puffy).
EDIT:
Looked it up
here.
GroundAutoLethality[Armored]
GroundAutoLethality[Hard]
GroundAutoLethality[Soft]