!All attacks and the fight itself are presented in the video, for those who are too lazy to watch, here is the strategy for victory!
Strategy :

First : Learn his moves. You need to know how he attacks in order to escape from death.

Second :​ Find an efficient way to deal damage without taking too much of it. Here are the moves I suggest for each attack :

Bats​ : Dash into the second one (with Shade Cloak) then jump to evade the last one. Then, spam your attack button (you can hit him about 3 times).

Dive​ : While he dives, you must be headed in the bigger half of the map. Dash in that direction then jump and hit him (once or twice) by targeting below you with your Nail.

​Dash uppercut ​: Run in the opposite direction, and watch out for the flames ! This attack is tricky, so don't try to hit him now.

Flame pillars​ : Move and get below him during the first pillar. Jump, hit and dash to evade the second pillar. Go back towards him at the third pillar and hit him twice. Then dash again for the last pillar. Do not forget :​ There is a flame on the floor for each pillar and that flame deals two damages also. Jump !

​Fire urchin ​: Go to one side and don't jump too high ! You may not reach the ground when you need to jump some more. I recommand a small jump when there is only one fireball on the ground level, and a high jump (not a double jump !) when there is two. ​This attack may stress you more than you was.' Stay focused !'

Do ​only ​heal when he is in bat mode, or else you may be loosing more masks than regaining some. If you are at full mask when you get him into bat mode, hit the bat with the pair of eyes to continue damaging him.

UNRECOMENDED: Grimchild since during that boss it does nothing.