I was initially trying to make this look kind of epic but it looks like these goofball villains were trying to come up with a profile pic so they don't get seen as a bot.

Might update with "boss music" associated for each of them.

EDIT: Just came up with ideas for boss fights but still haven't decided their themes yet.
EDIT2: FINALLY figured what their associated themes are.

Mutabalis, Food Demon: Gets stronger and heals from the food he eats, but every now and then, undergoes a transformation that leaves him open for attacks (Difficulty: Easy)
Demon Fight Theme: https://youtu.be/KS_6_6P3yaQ

Parish Dust, Weather Demon: Mostly airborne and launches attacks from the sky while riding the worst-looking motorbike (Difficulty: Easy)
Demon Fight Theme: https://youtu.be/009nFi292xA

Houdini, Ghost Demon: Must survive for a certain amount of time against ghosts whose attack are different based on how they died (Difficulty: Medium)
Demon Fight Theme: https://youtu.be/Gu9LOLo72Eg

Outslayer, Anatomy Demon: Hides and comes out to attack outside Oxy's line of sight, while his monsters roam around & attack; basically horror-themed Pac-Man (Difficulty: Medium)
Demon Fight Theme: https://youtu.be/opUoLgcsryU

Qrispy J./Maximizer, Sleep Demon: Immediately Oxy to sleep, putting him with dream layers with different attributes (Difficulty: Hard)
Demon Fight Theme: https://youtu.be/5L7MN2-xC10

Bolide, Judgement Demon: Oxy has to defeat him under a time limit before the area gets obliterated by an apocalyptic attack (Difficulty: Hard)
Demon Fight Theme: https://youtu.be/XEqT2QUD3ls

Andromalius, Infinite Demon: Lures Oxy into a wide, grassy plain that he can't escape from unless meeting certain conditions (Difficulty: Hard)
Demon Fight Theme: https://youtu.be/i5QvkUVboDo

Shax, Pillager Demon: Just messes and flirts with him; everything turns into a dating-sim (Difficulty: Impossible)
Demon Fight Theme: https://youtu.be/CjDFiFh2DJ0