^^ I screamed impossible nearly 50 times trying to beat that part. The best part about it was that after 50 minutes of trying to beat it, I finally beat it, but then I died and it never game me a checkpoint. And THEN, I beat it again in another 30 minutes, but I have no patience and after I die, I press start and Revert immediately, but I accidentally RESTARTED the level. It was the most pissed I've ever been playing Halo. I learned the key though, is to get on the turret furthest from the door. I was originally just trying to outshoot them by myself. I started getting on the turret and beat it pretty quickly then.
And yeah, you definitely want to take out the Watchers, the flying things first. I think they revive knights?