I have made quite long holes with hole saw by chiseling out the core. Two inches deep screw heads? do you have resipicating saw saw and a good metal blade? Cut V to weaken the treewood and cut most of the screw out. That kind of screws are often pretty hard and resist pliers and hand tools successfully. Angle grinder would do the trick in no time, but we don't want to burn that garage.
Anyway, weaken the bugger with any means necessary and then prong it up as Andrew says. I would have lost my temper early on, put old chain on my chainsaw, open the garage door asked miss to have a coffee/shopping break and made cuts just centimeter away from the screw both sides. Then mallet to beat the bugger into submission.
Main thing is not too loose cool and swear profusely and slip the mallet out of sweaty hands trough the wall, hedge, and BMW next door.
Pekka