If you've got a set of allen keys with a 90* bend , use the smallest one as a pick to remove any hard packed dirt from the knackered bolt . It's surprising how much can get lodged in there.
When you think it's all cleared out , try again with the correct sized allen key to see if you can get a better grip ( hopefully by-passing the chewed up bit to get a good enough purchase to enable you to remove the bolt )
If that doesn't work , try a non-metric allen key , tap it in place ( if there's enough room to do so ) or force it in as best you can , apply pressure IN with one hand and tap down with a rubber mallet with the other
Cheers , Stu