Chained Synth Summoner equipment/loot?

The party I'm DMing for will soon face off against a 15th level chained Synthesist Summoner. I've built the character and I can see why people have complained about it being too OP - without ANY items his AC will be 33! My dilemma then is this - how do I provide loot for my party from this encounter (as they are quite reward oriented), without making this abomination so much more powerful at the same time? Since he is likely to escape anyway, I was thinking of there simply being a cache of goodies left behind, maybe an altar to some evil god made from some rare material or some such. What are Your thoughts, reddit?