[beatcad]

that tool is available at home depot or sears for about 20 bucks.
but ya have to use it rite. you cant just whale on it. ya gotta put a little pressure and twist on it with it set in the rite postition(loose or tite)

in a pinch sometimes this will wirk with just a phillips screw driver if you can soak it(the bolt) with some PBBlaster for a few hours or overnite. i little pressure on the screwdriver and a good rap with a hammer mite "shock" it loose. i'll usually only do this on a part in a junkyard i just wanna get home, but it does work sometimes.

 

[beatcad]

i have also had better luck if you can get a small dremel cutting wheel in there and slot it so you can use a flathead impact to get it out....

that would be the next step. or a large head flathead screwdriver with a hex pattern near the handle(like a mac or snap-on) so you can put a wrench on it to get some real torque out of it. or a craftsman that is square so you can put a vice-grip on it.