This took me forever to find so Im going to help you guys/gals. After you rescue Cornelius from the Dungeon you have to go rescue Edmund and Trumpkin from the bell tower, so if you go down the hallway with the picture that you can destroy go left and than right up the staircase, go all the way up and pick Susan or Caspian and shoot the chandalier which will kill the two guards.
After that go down the stairs and go to where the guards were, if you were like me and went left and came to a door with no where to go (you can pick up the keys) go directly opposite of that door and you will see stairs leading down, take them.
Now you basically just have to battle your way through waves and bell people, after the first floor go up the stairs and you have another wave with bell tower people on the second floor, shoot them with your bows. Now a big guy will come and will rush you, you have to stand in front of the wooden door he came through and have him rush into it 3 TIMES. Now go up the stairs.
After you get all the way upstairs you will see Edmund and Trumpkin fighting guards, dont worry about them now, there will be bells and anvils? around the middle hole, push the anvils in by pushing B and tapping A, there will be 3 anvils. If a big guy comes with lots of armour on, go to a bell and push B when he is close and it should kill him. After you get all 3 anvils pushed off go join the fight with Edmund and Trumpkin, after every guard is dead, achievement will pop.