Vise-grips would be my first attempt. They even make ones with 'v-shaped' jaws that are particularly good for this purpose. The needle-nose ones are good for tight access.
Sometimes you can drive a tight-fitting 12 point socket on a stripped head and get some bite. Sears and others sell a set of sockets that have essentially left handed 'threads' inside just for this purpose, though I have not tried them yet.
You can also use a Dremel or a file to grind 2 sides flat on the head to receive a wrench, or a slot in the middle to receive a flat-headed screwdriver.
Larger bolts can sometimes be removed with a chisel (use chisel to 'dig' a slot on the side of the head, then change angle to force rotation)
Best of luck.
1986 300E 5-Speed 240k mi.