Lord of the Rings: Return of the King Requests

On Minas Tirith: On Top of the Wall, how do i do it? the overrun meter keeps going over and I do not know how to do it except push the ladders. Also the Pelenorr Fields are really hard at the end when you shoot the witch king cause I have to kill the Mumak and the Witch King at the same time! Plz answer

how do you get invincibilty