The Destroyer should have a lower attack damage, more accuracy (so the lasers focus on your position instead of being spammed wherever the hell), and resistance against piercing projectiles except minions, maybe 50%. Make the boss less polarized than “use pierce or struggle”.
Skeletron Prime should have less HP (maybe 25000 -> 20000) and defense. His cannon should be more accurate and have higher velocity, and the cannonballs should explode on player contact as well. The saw and vice could be made a bit more aggressive. Prime is easy and takes forever, and it should be the opposite.
Golem is really deserving of a lot more. There’s a lot you could do. He could definitely use basic stat buffs because he’s basically immobile. He could also have new attacks. Maybe he should inflict the unused Dazed debuff with his melee attacks, and make his hands pop off, become invincible, and attack from midair when taken off similar to his head.
Lunatic Cultist is very fun, but it’s really easy as long as you know how to hit the right cultist. I’d move the Phantasm Dragon to his usual attack cycle instead of a punishment for hitting the wrong one (you’r still have to deal with clones if you hit the wrong one). He should also have more stats and projectile accuracy across the board so he acts almost like a strength test for the Pillars and encourages you to do the plethora of post-Golem events.
Finally, a lot of the Pillars need rebalancing. The enemies are often considered infuriating to fight.
For starters, after the first run of the Celestial Pillars, the amount of enemies you need to kill should be halved. Running through the Pillars over and over again to kill Moon Lord gets tiring.
Solar Pillar’s ground enemies should be slower and have less health and knockback resistance so you can fend them off on the ground without having to fly. Corites should be able to be knocked out of a dash. Crawltipedes should have a higher target limit, maybe 20 blocks off the ground.
Stardust Pillar and Vortex Pillar are fine, except Alien Hornets should travel through blocks to stop cheesing and Flow Invaders should be less tanky.
Nebula Floater projectiles should be slower and brighter, and they should have much lower health. Predictors should have less of a wide spread.