I've been there too - a few times! Try a hammer and chisel, not an air chisel. Strike the bolt along the outter edge in the counter-clockwise direction. The shock will usually break it loose.
I found that a Craftsman 3/8 drive 6 point 1/2" socket fits this bolt perfectly. I have not rounded one off since.
Experience is something you don't get until just after you need it.