I used to be fairly terrible at Oryx for a long time, I eventually got pretty good at it (never like the OP, he sounds legendary) but realized the DS4 was simply not for me so I switched to M/KB and improved even further.

The real issue was that I would always struggle with sniping knights which soaked up time that could be spent doing other stuff like killing the eyes. There's probably a lot of people that have the same problem - they just aren't good enough or practiced enough to be capable of doing all of that.

Now that I changed my setup to M/KB I'm pretty much wrecking Oryx all day, killing acolytes, multiple knights, eyes, centurions etc... because I am much more comfortable with the controls now, confident that I can react faster, and able to process the situation better. Honestly it isn't that scary, similar to the Spindle mission where you simply have to destroy everything in the last room before it gets out of hand and when you do then everything is 100x easier.

If you have trouble taking care of things on plate duty then here are some tips to help you train up:

1. Tell your teammates that you are riding the strugglebus so they can help out.
2. Ask for plate 1 so you always have someone across from you.
3. Get in a safe position, crouch and take care of your knight, then throw grenades at the adds and use rockets or MG to help clear adds. Use synths as needed. Save your nade instead of using it on the Ogre if you need to.
4. If you miss your Knight just wait for him to cross the middle and use a rocket to kill everything. If you did #1 then the other guy will be watching and helping so you should be okay.
5. If you don't kill him there (somehow) call out that you leaked and try to kill him before he detonates - but know that this is basically a failburger and you'll need to practice to get it right.
6. Spec for high discipline if you can so you'll have grenades more often.
7. Don't use ToM on a plate (unless you can pop BoL every round) because it's probably not as good as your normal primary and you don't even need it to fulfill your role. You'll be more comfortable with your regular primary and that is more important here.
8. Don't panic when the guy says he has the relic - you don't have to evacuate from your plate! If you're still recovering from damage its okay to wait until its safe to run back to the middle. Look at the situation and do whatever is safe.

Obviously the above tips are only a bandaid on the real issue which is a lack of practice/skill and I don't recommend using this approach unless you are just really struggling... but doing that can help you get practice on taking your knight out and once you can do that consistently then everything else is going to be really easy. You'll understand the timing of their spawn, their positions, the adds that you actually have to worry about, where the eyes appear, all that jazz.

That being said, the correct approach is this:

1. Destroy everything.