I've used a portable air tank and an automotive style impact driver.

If you don't have an air compressor big enough (or don't want to spend
the bucks for a nice air tool), you can get an electric impact driver
from Harbor Freight Tools for about $100 (they have a weaker one
that's even less). Anyway, the thing is massive and must weigh 20
pounds but mine has yet to meet a bolt it couldn't shake loose (even
transmission mounting bolts with plenty of permanent red threadlocker).