Hieronymus tl;dr

• There's a Green Lantern. Heal it to full, and it'll shoot a fireball at Hiero and kill itself and get a 50% healing debuff for 20 seconds. Heal up to 5 times for bigger damage numbers.
• There's a Purple Lantern on INS and above. Shoot it dead and you'll get bigger damage numbers, but will nerf heals on your precious students.
• Hiero has lots of DEF. DEF down students will come in handy.
• Everyone will take constant chip damage, even the Green Lantern. Plan accordingly. Don't heal the lantern 50% and expect it to stay 50%, and don't leave Light students unattended.
• If a student gets cursed (big number counting down from 10), either heal her to 100% or watch her explode. Sometimes it's fine to let her explode, especially if she's Special or Elastic and nobody else is near her.
• If Hiero does the FMA, you've got 10 seconds of invincibility to heal your students after the attack. Otherwise, they'll get KO'd from the aforementioned chip damage.