I just looked up what I did to get my Sierra 6.0L bolts out when I removed the compressor and mount.
15mm open-end on the bottom rear, flipping every 1/16th of a turn (like a line wrench).
I know thats a different bolt, different vehicle, but maybe that'll be helpful. The comp. and mounting bracket are in the same place on both, at least.
Also, mark the bolts / locations, if needed - they're different lengths, and there's nothing like finding out you put them back in the wrong spots, when you get to the last one and find it's too short / long.