This is actually quite a simple battle to win, it just requires a bit of preparation first and you're golden.
This is the last battle of the game and there's no save point afterwards, so there's no reason to hold back on using items, so make sure to keep your life topped up to maximum and keep an active Stamina Drink effect and you should make it through fine.
The big issue with the fight is simple: the Dark Genie acts like a parade balloon and floats around overhead the entire time, never once coming down to the ground just flying back and forth across the battle map, meaning the only way to hurt him is with your person's ranged attack.
What makes the fight challenging is how he will spawn about half a dozen or so minions that constantly try to punch you or get you with a ranged goo attack. The Dark Genie himself will use his only active attack rarely, a massive orbital laser purple beam of death that sweeps across the centre of the area and can one shot you if you're unlucky enough to be hit, so having Resurrection Powders equipped is a must, as well as a handful of Anti-Goo Amulets.
The trick to the fight is that you can only hurt the Dark Genie while it's spawning minions, so either you have to try and actively kill these annoyances as they constantly swarm you (and deal with the problem that the game won't let you lock on to anything but the boss himself), or you wait for the massive purple beam of death to sweep the field as it damages the minions as well.
I preferred to use Monica as her ranged magic attack is a homing missile, so it doesn't matter what direction she's facing as long as she's locked on as it means she'll always hit.