Firstly, it's extremely unclear how to damage it, while there's a reliable method for doing so you do a very poor job at conveying it. The player needs more feedback in game to know that dodging the bosses attacks by staying as far away as possible isn't the correct way of dealing with things.
Any suggestions relating to how I could convey it? I'm not being sarcastic, I honestly am not sure how the player would figure it out if not by simply seeing the pattern after the boss attacks enough times. Maybe the boss should dash away after it attacks like it wants to get away from you?
Also, because the player is encouraged to stay near the boss to do damage, the player needs warning as to what attack is coming up next before it begins. This is fine when the boss is going to fire the THZ turret because of the audio cue from the charging noise, but the other attacks come out without warning.
I suppose each attack should have a different charging sound. Perhaps the turretfire should have its current sound effect, the direct-fire energy spheres should have the S3&K charging sound, the rotary-fire spheres should have the Spark Mortar charging sound from Mystic Realm, and the final pinch attack should have all three at once.
The pinch phase is unreasonable and cheap rather than difficult as well. The attack where you are shrunk is fine imo, but the combination of rapid fire missiles and THZ turret spam in the second attack is absurd. Also, the method used to damage the boss during the pinch phase is too difficult, even for what is intended to be a final boss.
In the case of the combination attack, the faster turretfire should probably be replaced with the slower variety (if you didn't notice, there are three types of turretfire, each with a different speed). Perhaps, for the damaging method, the boss should wait a couple of seconds after the thrown shield explodes to generate a new one, widening the vulnerability period.
I have to ask, though, where the boss would fit in the level pack if not at the very end. It doesn't seem like it would fit at the end of Haunted House or Steam Engine. If Dyson Egg's full act is included, it could be used as the miniboss of that, although from what you're saying, Whackjood, it won't be. Even if a place could be found for it, though, I'm not sure it would function that well. The boss's difficulty and overall pattern were designed around the idea of it being the final boss, and it would have to be dramatically altered to the point where it would be easier just to make a new boss if it was downgraded too heavily. Any ideas, Whackjood?