Really? I did it in two tries, making sure I picked the right turret and using bursts to ensure it doesn't overheat. If you're quick to aim it's relatively straight forward.
I'm really enjoying the campaign on Legendary solo, makes each battle much more about pitched mind games and prioritising the right enemies.
Still need to do co-op Legendary (Lone Wolf just doesn't seem fun, I'll be honest).
I've been doing a mission a week to go with the challenges and I've just finshed Chapter 5. The only bit I've been stuck at for more than 3 retries so far was on Chapter 4 where you're supposed to hold off a Promethean onslaught with some marines while Cortana opens a door. Naturally all the marines were dead by that point on Legendary and trying to hold them off with a turret just doesn't work so I had to do some very creative hiding to get through that bit.