Constantly kill the attacking orcs to keep low your meter. Every time a ladder appears, kick it down. Do not let the orcs overrun te wall. Once you reach the point where you must stop the siege towers, use the catapult to take down two of them. Also use missiles and magic to stop them.

whin you get over run go to s spat where they cant find you and stsrt shuting your missles
if you run out keep pressing A and run out whin to towrs come us the catapleed win one gets to the wall that after you beat it you win ps stay far enough to get the target on it and shut like crasy

i think u should unlock faramir using a,a,y,y then do what the others told u to do