Ridiculously hard, I would like to make a Legendary FAQ but I don't understand how I'm supposed to beat this mission. I normally jack Wraiths with my Spartans while using another one and the Omega squad to destroy the mega turret.
But the Mega turret can fire at units right next to it, normally destroying my force before it can get it down to half health? The Cobra is practically useless, gets destroyed straight away. You can't turtle because of the mega turret and there's no way I can pump out a force strong enough to go destroy it without leaving my base totally defenseless.
I had no idea about that but I have found a way to do it (atleast, I'm 20 minutes in and just started constructing my base... Lol)
The Covenant DO NOT attack you once you've built next to the turret. Send all your units over, clear it and destroy it ASAP then build a base next to it. I had one Spartan left, and had to collect crates to construct a supply pad but it gives you time to construct a base. Forget about the crater, defending it is hopeless.