In to3 (and I've been told other bosses too but dont know)
Story mode. (not sure about dungeon)
For the three major bosses if you redeem or rejuv or buff before they are attackable the buffs are applied to the boss and not to you.
For viridian, when he becomes invincible with his little minion, if you have him selelcted he is healed if you use heals and buffed if you use buffs.