Guys, go to this thread Better Cavalry Management if you want to be able to kill Mumakils with your cavalry units.
This really helps in killing them with factions like Rohan/Gondor, as your cavalry are now your strongest unit (Aside siege equipment) for Mumakil battles!

IMPORTANT!!
READ THE INSTRUCTIONS IN THE THREAD ITSELF ON HOW TO BACKUP YOUR FILE. IT IS QUITE IMPORTANT.