I actually played through the entirety of Halo 2's campaign on Legendary until Tarter Sauce where I gave up
Watching speedruns really helped. Able to glitch through some of the maps.
I only play on Legendary because anything less is cheating yourself. Embrace the pain, the fear, the need to actually seek and cling to cover, or rush an enemy and bash his head in, or DIE.
talking like this makes me want to pull out Reach and strap on a pro-pipe.
(I had to quit for a month and come back before I could beat Tartarus. I had refused to look up any hints or tips, but I finally gave up. I can't remember how I beat him, I think I had to lure and kite him or something. But yeah that fucker was almost impossible on the hardest mode.)