Need help with strategies to defeat commander Niall

I’m playing Elden Ring (after beating Dark Souls a few times and 100%ing Dark Souls 3) for the first time and Niall keeps beating me with his billion AOE attacks and I need some tips on how to defeat him.

My main weapons right now are the Iron Balls in my right hand or two handed for jumping attacks and Godrick’s dragon in my left. I’ve been opening the fight by hitting Niall with the awesome dragon attack before dashing away and using Mimic tear before or after taking out the 2 sword guy. Then I’ve been leaving the shield guy alive as long as possible so Niall doesn’t become Super Niall on some Goku timing. Once that transformation hits everything falls apart and I can’t find good windows to take him out without dying. Any tips and/or tricks on what I should do differently and what good attack windows are?