Boss Battle Concept

The enemy boss is a mage.  In the build up to the battle, he summons a breakable magic wall.  He does this to buy himself enough time to summon 4 dopplegangers of himself, which attack once the wall breaks.  Immediately after, he chugs a strange purple-ish green potion which makes his body shake and give off green steam.  He has chugged pure magical energy, more than he is ever used to handling at once.

He then summons a forcefield around himself, putting all the mana he has into it.  The forcefield is nigh indestructible, save for a mana-absorbing dagger the protagonist obtained earlier.  Once safely inside the shield, the mage chugs another potion just like before, letting out a painful scream.  His eyes explode, and the mage falls to his knees.  He then begins using the potion's effects to imbue the ground with magic.

His plan is to destroy the environment they are in (a castle, perhaps) by detonating the imbued magic.  His posture functions as the amount of time the protagonist(s) have: standing straight with arms stretched up -> fallen to knees, arms still held up -> hands and knees on ground -> collapsed on ground, only propped up on elbows.  In the last stage, when the mage's hands turn red, the player has 3 seconds to eliminate him or else GAME OVER.  Naturally, as more mana is imbued, the shinier the ground in the entire area becomes.  Tthe battle has a hard time limit of 8-10 minutes.

As he is doing this, the 4 dopplegangers engage the party.  Each of them adopt different roles: one does rangespam, another does melee, one focuses on defensive roles, the last one starts as a status effect harassment but if any other doppleganger dies, he takes their place AND has an extra 50% efficacy at the role (i.e. 50% extra dmg if melee, 50% faster healing/defense if defender, 50% more ranged attacks if rangespam).  As more dopplegangers die, each one becomes more dangerous (more bullet hell, faster melee, etc).  Should the healer or defender be the last one standing, they switch to rangespam and melee, respectively.

Even if the player kills the dopplegangers, if they can't figure out how to break the superbarrier... gameover.  But if they do, the story would progress.  The next step would be to de-magic the ground, the difficulty of which is dependent on how long the battle took.