It's been a while since I last played this game, as I remember, in the last battle, at first you will fight him a little, then he will teleport to the balcony, and resurrect the 9 people you killed, so you have to kill them to, then he will come down and duplicate himself, and when you strike the real him, he will teleport lower in the area, down the stairs, just fight him until Altair shoves his hidden blade into his throat. Or, you can speed things up, and as soon as Al Mualim falls you can quickly change your weapon to the hidden blade and strike him, I hope this helped