Some strategies, in case some people need them:
-use Black Knight weapons:
-they deal 20% more damage against this boss
- they deal high poise damage, allowing you to use critical attacks more often
-Use resins for more damage
-only summon 1 phantom. I would recommend Lapp, but its up to you
-bring Moss against the toxic. Otherwise, the fight might get very, very hard
For Phase 1:
-run into the demon that is in the "fire mode". Dogde his attacks by simply staying near his legs and just keep attacking them
-if possible, keep an eye on the toxic attacks of the "burnt out" demon
-if you use great weapons, use the critical attack when possible
-if you don't, just keep hitting when they are in a vurnuable state
Most important of all:
KILL DEMON FROM BELOW LAST!
Otherwise Phase 2 will get a lot more difficult!
For Phase 2:
Same strategy as against a "flame mode" demon.
-Stick to it's legs or his tail to avoid melee attacks.
-roll backwards or to the side against fireballs
-roll sideways if he uses his flying assault.
And in any case, just keep on attacking him, but always have enough stamina to roll once or twice.