Walkthrough
Elden Ring: Gelmir Hero's Grave Full Walkthrough
Elden Ring progression guide: The best order to tackle areas and bosses
Elden Ring walkthrough to complete the critical path
Your best bet here is just to run and try and get behind him before he can start spreading his flames.
Attack from rage if you can.
The area ahead is a super dangerous courtyard, filled with Exiles manning deadly weapons.