I stripped one myself. Haven't figured it out yet, I'd love to hear an easy answer.
I did learn that the bumper cover isn't all that tight on the core. It's only like four little plastic press anchors that hold it on. You can always drill new holes if you take off the plastic bumper strips (but that can split the bumper cover). If you didn't want to use the old fashoined easy outs...I'd be tempted to pull the lights & cover off carefully, then chop away some of the bumper core to get at that bolt with vice grips or something.
That method might work unless you cracked the bolt off internally. Then I'd go in with the easy outs. Pain in the ass, but it's reliable. Just keep drilling marginally larger holes with a lot of oil to cool the bits. Don't bend them either. If they snap you're totally screwed.
Repair any damage with plastic epoxy or even JB Weld & sanding.