Law, the only easy one is one I wont even recommend. If you search, you will find people who put a breaker bar on the crank bolt and then they disable the ignition and fuel system and bump the starter. I WONT recommend that for the very obvious reasons you can see, but some folks do it that way.
Otherwise, finding a way to hold the motor from turning over is about it. have you tried to push the breaker bar real quick (rather than slow and hard)? That kind of mimics an impact gun. Ditto for hitting the breaker with a 4# hammer. When I have a smaller fastener that is stuck and that cant be leveraged out, I will pound on the wrench with my fist. This pounding breaks it loose. Same idea here.
Guys use prybars, etc to wedge stuff up, etc. Or put it together and find a close shop that can blap it with the air gun for you (probably for free) and then tighten it just enough to get home.
I would say you could get a cordless or plug-in impact gun, but I have never used one so I dont know how well they work. I did once use a 12 volt (plugged into the cigar lighter) impact gun (20 bucks at wally mart) and it actually loosened lug nuts.
That is all I can think of bud. Good luck!