It looks to me like you have enough room to get a normal cordless drill in there, no? If so, there's two ways to deal with it:
1.)Get a set of left handed drill bits (screw removers). Take a normal drill bit and drill a pilot hole and once you have enough of a cavity, insert the left handed bit and reverse the bolt out
2.) if you bugger it up using the above method, all you have to do is just drill out the center of the bolt. Take a small drill bit and drill all the way through until you are at or near the bottom of the bolt. Once you have that 'tunnel' drilled, you can move up to a bigger drill bit and remove more material from the center of the bolt and down. Progress over two or three sizes of drill bit will get you closer to the later edges of the bolt (careful not to bugger up the threads of the fender hole). Once you've removed enough material the bolt will lose tension and you can spin it out with a left hand bit or even a pair of needle nose pliers
If you CANNOT fit a drill at a 90* angle - this is what you need to do:
Drill a small pilot hole (dimple) somewhere near the edge of the bolt shaft - this dimple needs to be just deep enough to accomodate the pointy end of a punch like this:
Once you can get the pointed end of the punch in the dimple, just position the punch at like a 45* or 30* angle and keep tapping the end of the punch with the hammer in the direction of loosening the bolt (to the left). This will take a bit of time, but once you start the bolt moving - you are in the clear and it will spin out easier
If all else fails, go to Sears and buy a angle drill and do Procedure 1 above. If you go to Fatwallet.com you can find some good coupons that will get you a Craftsman Nextec angled dtill for less than $50.
Lastly, make sure to douse whats left of the bolt with Liquid Wrench (the yellow can) before you start:
You'll have this out in no time.
2011 Mustang GT M6 3.31s
Matt HONEYCUTT Tune, Lethal Off-Road X, GT500 axlebacks, J&M LCAs, UPR UCA, Whiteline LCA relocation brackets, Eibach Pro-Kit Springs, Koni STR.T shocks/struts, GT500 strut mounts, SR Strut Tower Brace
Best 1/4 E/T (05/18/14 MIR - no tune, X-pipe, or UCA): 12.920 @ 109.40 mph (2.059 60ft)