The hardest content for me in any Halo game ever is still the Heretic Elites setting for Firefight in Reach on Legendary.
It's basically AI using human weapons with zero restrictions. Also, then they use projectile weapons like the Rocket Launcher, the AI rockets
home in on you, even when you're just on foot.
Heretic waves + Installation 04 I only ever completed with massive, massive amounts of cheese. In the rare spots it showed up in matchmaking it was guaranteed to wipe everyone out and end the game.
Ghaleon and I got bored once while we were hiding
during a Heretic wave, and we turned it into a game of who could survive the longest going out the door. I would jump out and not even hit the ground before I was dead.