Actually, now that I think about it, in the Flying Fortress you learn two things:
Mirror Movement and Perfect Blocking. The Sentry seems like it would be the perfect spot to be forced to use either one during a boss battle.
Eight mirrors around the edge of the room, four in the corners unable to be moved, the four N/E/S/W on straight indentions.
Phase 1: Sentry is bubbled, spawns in enemies rapidly up to # enemies (like Toy Machine).
Phase 2: Enemy waves wiped out, Sentry begins pulsing with light. Spits out 1-3 rapid phaseballs every 5 seconds.
Option A: Perfect block the balls back at it to knock off it's bubble and damage it. For speedruns and getting Arcade S-Rank. Auto-breaks shield if the perfect block is failed.
Option B: Push the mirrors into a position where the player can stand in front of one, bait Sentry into shooting at it and they can bounce-bounce-bounce to hit Sentry.
The 4 mirrors on paths shuffle to a random point, enemies continue to slowly trickle in while Sentry's bubble is gone. Until Sentry hits 50% HP, then...
Phase 3: Sentry re-bubbles. Enemies now spawn as groups, instead of one by one. Sentry now occasionally fires an orb even during the enemy phase. Wipe out groups, avoid phaseballs, get the mirrors back in position to knock off Sentry's bubble again and finish it off. Or just perfect block again, if you can.
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Incorporating the mechanics of the dungeon would make it so much more fitting and interesting. <3