phase him ? after he reaches some % hp point replace him with "phased" form that has the same % hp/mp value and faces the same angle
for example the phased boss equips a weapon, grows tentacles, bunny ears whatever
even though it's pretty easy to do with a single unit you can add/remove skills anytime and alter it's stats by simply putting boost items in his inventory like
(core 2 unlocked - item +40 armor +50 attack +attack speed) or just alter the hero's stats directly (if the boss is a hero unit)
that being said I don't like this "willfully weak" behavior it's stupid and unrealistic IMO
let's take wow, if you can't beat a boss in under 8 minutes he mans up and rapes everything in enraged mode, I mean seriously ?
couldn't he man up on the start and just rape everything to begin with ? it makes no sense
I could accept that his power is suppressed and these "seals" get removed/destroyed/intentionally unlocked during the fight
or if the enemy is too strong (lorewise/storywise Idk) then make the players match him in power
I mean did you see the flying deathwing fight ? that's the most stupid game mechanics> realism fail boss encounter of all time
cutting scales off a gigantic dragon's back ? *yawn* can't he freaking do a barrel roll or something ?
if you want to get some inspiration monster hunter has some great boss fight designs (they also get this enrage mode that does not break the balanced completely down)
now you said that even while willfully weak he is still very strong
then by the end of the fight all the debuffs are going away ? you will have hard time making something very strong
that has to be much stronger by the end of the fight, without breaking the difficulty and making him unbeatable
and if you make the buff too small the "willfully weak" phase won't be very credible
you could make it so that when he goes "strong" the objective will be to stay alive until a set amount of time
or just make it so that despite he got a significant buff in strength his other sides get nerfed down
(significantly higher rate of damage-taken, make him move slower for example, reduce the AOE range of his skills/raise cooldown of his skills, higher miss ratio etc)
that or some kind of interference will appear (maybe he grows in size and obstacles on the ground will obstruct his movement ?)
or due to his newfound power a new potential threat will appear ? (rocks/ceiling falling down because of his blast waves and damaging him
or his "slam" attacks breaking the floor making him fall a few stories down, or let's be realistic lol lava will gush out of the cracks damaging his feet)
but interesting if you pull it off *thumbs up*