Citadelle boss fight

I need some advice for the boss fight. This damn thing has me at my wits end. Typically i'm going itno it anywhere between 20-25. I've been taking perkaholic, Power Keg, and arsenal accelerator into the fight. I've also been bringing the Lion sword with me as well for the armor recovery. The gun i have been using is the GPMG-7. More recently, i've been able to get him to the final stage but that arena is so damn small on top of the fire he spits on the arena and having to dodge the regular enemies as well. For the love of god, any advice would be appreciated.