I agree that making bosses immune to status effects is awful, and I think a lot of RPGs have found good ways around this. My main gripe about it is that it makes certain classes that specialise in debuffing entirely redundant, and I don't think any fight should render someone redundant. If they can change jobs it's obviously not so bad, but yeah.

I think it is entirely fair that a boss be immune to instant-death spells, because otherwise you can just keep repeating the fight until you get Selphie's The End spell and just be done with it, which was always a very, very cheap way to beat Ultima Weapon in FFVIII. I do, however, think that bosses should be susceptable to the likes of poison, slow, deshell, and the like. Things that aren't overpowered, effectively.

I find FFXIII shows great examples of how bosses can be varied while still allowing buffs/debuffs. Sometimes they are key to victory! Also, even Orphan itself could be damaged by debuffs. The counter is a high HP count, a method of removing debuffs, etc. I remember fighting some boss in one of the PS1/2 era FF's and they would constantly dispel debuffs and it was frustrating as hell, but I still had someone keep doing them because it really helped my damage output. For example, if you have a mage, they might be hugely resistant to magic attacks, but very open to physical attacks when deprotect has been cast. With this in mind, casting deprotect constantly gave my party better damage output overall than if I'd stuck with regular attackers. I like teamwork!