Yeah, the Haigure move of the Haigure boss (which can only hit megucas with full Haigure) replaces all moves with a useless Haigure skill. The other Haigure moves aren't that bad, especially the stackable Haigure buffs. The boss is beatable with those Haigure moves if it didn't use that Haigure skill.If you have maxed out Haigure then you can't use any abilities but the 4 Haigure abilities which aren't that strong, the boss will also use an attack which will lock you into using a new ability which does nothing, effectively making that member useless until all 4 are useless.
I updated the Haigure boss in the new Haigure build on Itch. It will only use the Haigure skill once, and has slightly lower stats. Even with the original stats and full Haigure on the megucas, the Haigure boss was beatable if it could only use that Haigure skill once, but this should give some more leeway.
I also added some (true) anti-Haigure items once you finish the Haigure dungeon.