This mission is actually almost fair, with you and Richard against Saladin and Sultan. You have a monetary disadvantage but the biggest pain is that Saladin stands on a mighty hill surrounded by resources while you don't have very much at all. Since Saladin is the money making king and also a turtler, you don't have too much to fear from him at first as long as you don't forget your defenses. My biggest advice would be to place a square tower as close to Sultan as possible and load it full of archers and a ballista or mangonel before he can get his defenses going to waste his money and effectively cripple him. Watch out for Saladin's siege engines and superior height and you shouldn't have much problem defeating him. Ask Richard to help you attack whoever your current target is and he will assist quite handily.