It took me awhile to figure him out.
You have to stagger him to get the big damage or you'll run out of ammo. Here's some things I noticed fighting him
- Boost forward when he lunges at you to attack. If you're fast enough you can use the charge sword. If not just use w.e you have that can burst stagger. I used the shoulder laser rifles there.
- When he does the two red missiles that explode napalm you can do the homing thrust while dodging them and attack with a melee weapon if you have it. You can also home in on him when he does the laser beam that sweeps and area.
- In phase two when he does the coral attack don't be on the ground.
My build was the charge sword, two laser rifles on the shoulder and the plasma cannnon that shoots purple lasers. The two plasma cannons create alot of stagger when you shoot them at the same time but the recoil leaves you vulnerable. The plasma cannon was used to keep the stagger bar up. I'd knock him into stagger with the dual plasma rifles or a single plasma cannon shot. When he was stagger I'd use the charge sword and charge the plasma so I could hit him with a full blast after the sword attack.