I wrote the below solution for the Legendary difficulty run, but you can apply some of the same ideas if playing on Heroic.
SOLUTION:
1. Use and abuse the nearly unlimited ability of sprinting so you can avoid a lot of fights.
2. If you keep dying while trying to do step 1, then you might want to consider the opposite approach and take the time to clean all the enemies that can do considerable damage against you before you can go through a door/passage/objective. You don't have to clean every single little grunt everytime unless it's necessary for the story to proceed. I skipped, for instance, most of the Battle of Sunaion level just running through. Other times, like the descent down the Guardian's body in the level called Genesis, I'd die everytime I tried to jump jump jump run run run. So I knew it was time to try a different approach, taking the enemies out with the Battle Rifle from a distance or even while in the air, and it worked.
3. Frequently push up on your D-Pad (
) towards an enemy to highlight him and command your team to help you wipe him out, 1 enemy at the time, specially the most dangerous ones and the closest to you, otherwise, if you send them to kill the sniper that's behind the whole enemy army, they'll likely end up dying. You'll notice that they'll help a lot more if you constantly command them against an especific target. You can also command them to change the weapons they're using by looking closely to the weapon you want them to pick up and highlight it. One of the 3 members of your squad will take it. If the wrong person does it, have them undo it and highlight it again. I couldn't decide if it was worthy giving them power weapons (such as the prometean rocket launcher) or if it was best to keep the spares for myself. If you're just passing through but can't carry any more, then I'd say it's best to give them up to the other members. Anyways, just make sure they have good weapons. Covenant ones (in general) seem to be the weakest of all 3 factions.
4. Sometimes is better to let your team die if you can't control them or pay too much attention because you're trying to survive. They will spawn back, although it takes a little while.
5. Related with the step above, when fighting the Boss, stay out of his physical reach as much as possible. For instance, in one of the open space fights, I jumped down to the ground where the "W guy" (trying to stay spoiler free) was only after I ran out of all the ammo I could find in the high ground caves I came from. So I took A LOT of his minions from afar. When fighting him in an indoors/closed area I kept him tagged so I could easily watch and avoid him, always keeping some kind of obstacle between us to cancel his direct hits and special attacks. In one of these areas I noticed later that there were special weapons (a prometean rocket launcher, for instance) on those high platforms that stay at mid-air, so that helps too, provided that you perform the jump to reach them.
6. Still about the recurring boss, I'm glad I could skip our 4th encounter on The Breaking as showed on this youtube video (not my video): www.youtube.com/watch?v=tP49F59FUDw
It's not that hard to perform the jump, it took me at best 10-15 tries and remapping the controller buttoms to make it easier for me to press them correctly. I think the video doesn't mention it, but if you're falling too short on the corner that you're trying to reach, you might want to try and run straight first on the small pillar you're standing on, just enough for you to jump to the side then, thrust and ground pound in the corner. If you don't fall exactly on the top be ready to try and grab the edge and climb up (
)
So, I never had to go through that fight, but I guess this is a HUGE help, frustration and time saver.
7. My final advice is for when you keep getting killed by the same enemies before reaching a checkpoint. It happened to me on the last mission with the blue team and it was a pain. The enemy snipers were getting me constantly, it seemed like a impassable frustration. In reality I was just tired and didn't want to repeat all the same other kills only to get another shot at the guys that were killing me over and again. But I had to, so I took my time, waited for the rest of blue team to respawn back a few times (always wait for all of them instead of sending just one alone to fight for you), as many times as it was needed and then started to mark the snipers real quick and went back to cover, ready my "Smart-Link" (what a fancy name) and then took them out 1 by 1, before rushing back to cover. Always stayed away from Knights, because they are also deadly and can overcome you as well as the party with ease.
Well, these were the tatics I learned while playing. If I remember anything more useful I'll update this solution as well. Feel free to leave a comment too and a thumbs, if any of it ends up helping you. Thanks!