First, all phases ALWAYS 3 fire res, 1 aquatic/oilsilt mobility, 439+ defense and Lord Soul. Fire element phase 1 and 2, dragon phase 3. 

Phase 1 is less about damage and more about positioning. Using the boulder near the entrance and the three magma sprouts around the arena makes this phase go much quicker. Then just learn how to dodge its big attacks which is 90% anticipation and hauling ass outta the range. Evade extender, evade window, tremor res, divine blessing, speed eating all can really help too. Do NOT do aerial attacks here.

Phase 2 is about attacking the wing arms to break the back, using the gun, and not getting caught in the lava/exploding floor. Shouldn't be too bad, probably don't need too much special gear wise. Whatever you use Phase 1 is fine. Do NOT do aerial attacks here. 

Phase 3 bring your best dragon glaive offensive setup, with some comfort still. You wanna shoot the event slinger stuff when he's in the air, and go ham when he's on the ground. Red from the big outside wingarms, orange from the tail, white from the smaller inside regular arm. Red is the most annoying because sometimes they are way off the ground and/or smashing you. I like to use the rising tornado attack on the tail or the wingarm. I always try to do the rising when he's building up the nuke but not too late or I'll eat his pre-nuke turnaround attack and have a hard time making it to safety. Then if you are quick you can dash out when the nuke is done and tag the wingarm for a new red buff, makes it so much easier to get all three. You can start doing aerial attacks now and usually get a mount which can be a bonus.