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You can reduce the defense to zero, so you can add armor. Although, it would be better to give a shield wall ability, so that the defense could be reduced, but it wouldn't be critical, because in the case of armor and 75% magic resistance, defeating the boss will be extremely slow and tedious. As for immunities, only immunity to mind control is useful for soloing the boss; berserk will work against a casting enemy in this case:), and blindness will be almost useless, and immunity to mind control doesn't protect against spells (there's a separate ability for that). Yes, there's also dispel, but again, in a situation where there's only one boss, it's not that difficult to remove the effect and attack (especially when he has 9 initiative, lol).