Trying to replace tensioner pulley

The tensioner on my 1999 Cavalier 2.2L is going. The casing is coming apart at the seam of the assembly around the bolt that attaches it to the car. I've got the replacement part sitting in the box, but I can't get at the bolt to remove the broken tensioner.

There is no access from underneath the car and the space between the tensioner and some unmovable piece of metal is smaller than my shortest 15mm socket. Reaching in at an angle with my open-end wrench is only stripping the bolt.

Am I going at this all wrong, or does anyone have any ideas or helpful hints for me to get this thing off?

Thanks for the help!

For anyone interested in following my drama with this thing, I got the bolt loosened up, but it is longer than the space I have to pull it out. Any ideas besides cutting the end of the bolt off? It is completely unscrewed and loose, but one end is still in the casing on the other side of the tensioner, and the other end is against a piece of metal welded into the car.

For the sake of anybody who may have this problem in the future, I cut the long bolt off with a hack saw (took a long time!) and replaced it when I installed the new tensioner. Back on the road last night with everything working (phew).