We get plenty of cats in at work with "stinky butt" fur. We usually wet the cat, lather it up with soap real well, and then let them soak so that it loosens up the feces. After that, we're usually able to rinse them off and run a small flea comb through and get the rest out. If It's badly matted, you may have to get some electric clippers to shave the area and just make sure you clean it more often.