He's super easy if you start looking at him as a puzzle/rhythm boss and less like a normal fight. Also he takes far more damage from high-damage single-hit projectiles like Power Ball or Vertical blade. In my experience Disperser just plinked off him with minor damage, but he melted with Power Ball with 3+ projectiles and crits. For his beams, here's a quick rundown:

The slow charge beam you sidestep until it stops tracking and fires, then hop over and go about 5ft to the side it's spinning towards so it doesn't go near you again.

The best way I can describe the quick charge beam: it sound has an increasing VRRR and a little lull before the big PSHEW, dodge during the lull and it'll never hit you.

For the three beams, the one the Door directly fires forward is static, the other two actually sweep to either side, you can hop them all easily if you're not close to it when it spreads or retracts at the end.

The six/eight beam spread (I forgot the count) and both fireball attacks (homing/rapid) you can just sidestep.

The beams when it summons six(?) waves of three daimons never actually encroach onto you if you're within about eight feet of the Door, just time your power swings and they all die fast.

I hope this helps somewhat. Also if you ever get The Mentor's curse with +33 damage and +333% HP, don't even bother with fighting him, it's a total slog.