Best way, for me, is to put an air ratchet on the head of the bolt and just keep turning to back it out. It's slow, but relatively painless. Once I have enough clearance, I put a box end wrench UNDER the bolt head and use it to help pull the bolt out while turning it. You may need to apply some pressure on the threaded end to get it started. I used a piece of wooden dowel to press on the threaded end.
Just be patient and do not let the air ratchet rest on the fuselage skin. It can wear a big dimple in short order.