Actually, i take that back, the AI does a decent job with the right guys. I played a battle where the Mongols were attacking with 2 stacks, and my re-inforcements were mostly consistant of 8 Muslim Archers, 2 Norman cav units, and like 3 armored sargs units (with a general) and they did surprisingly well. They had really low casualties while dealing some heavy losses on the Mongols, and didnt lose the general in the process (he didn't evne get involved in the battle like the AI does sometimes)