I'm not going to go into any in-depth detail with pretty pictures and such. I'll just post what I found so far and leave it to someone else to expand. This is from the perspective of a high level. People can feel free to add what they've found.
You have five columns. Roughly every 10-15 seconds they'll start charging up. You'll see 4 of them have a red orb and one has a blue orb. Run to the one with the blue orb. He also will pull you towards him throughout the fight. He also will push you away from him as well. If you're doing this as a high level, just kite him.
The key is to keep them separated. One has a red orb and the other has a blue orb. I personally felt that killing the one with the red orb was easier as the one with the blue orb spams heal. Have someone tank the blue orb golem while the others go for the red orb one.
I'll also add that I was able to stand in between them both and separate them far enough that the blue orb golem didn't heal the other. This could mean you only need one tank so long as you don't use any AOE that pushes them closer together.
Defeat the gray golems one by one as they get a green orb. They'll heal Onyx otherwise. For lower levels, I'm pretty sure someone can just kite Onyx while the rest go for the golems.
His attacks occasionally will stun you so I felt kiting was easier than straight up tanking. When you hear a whistling sound and a blue circle around him, he is powering up for his quake attack which will make rocks fall from above. These cause massive damage and have an AOE range as well. When he charges just stop attacking and run far away from him. The rocks fall in a large area centered around where he charges up.
I found nothing special about him. He just summons sandstorms over and over.