Pusillin

East: side chambers
Speak to Pusillin near the entrance before taking the upper route if the group needs the key. Follow his stops to the final platform, then work through the satyr side rooms and the pool. After defeating Zevrim at the altar, return to the Conservatory and have the barrier toward Alzzin opened.
Pusillin
Elite DemonPusillin's chase ends in an imp pull where Spirit of Runn Tum makes direct spell attacks dangerous to their caster.
Abilities

Fireball
Inflicts Fire damage to an enemy.
Interrupt Pusillin's Fireball when you can do so without casting a reflected spell into Spirit of Runn Tum.

Fire Blast
Inflicts Fire damage to an enemy.
Account for Fire Blast while collecting the accompanying imps; it adds immediate Fire damage rather than a cast to wait on.

Blast Wave
Unleashes a wave of flame, inflicting Fire damage to nearby enemies and reducing their movement speed for 6 seconds.
Keep ranged players and the healer outside the nearby Blast Wave so its damage and slow do not catch the whole platform group.

Spirit of Runn Tum
Pause direct offensive spells into Pusillin while Spirit of Runn Tum is active. Use weapon damage on him or direct spells at the accompanying imps instead.
Tank- Start the final platform fight with the party together so you can collect the imps before healing attracts them. Expect faster melee attacks while Spirit of Runn Tum is active.
Healer- Stand outside Pusillin's Blast Wave area and keep the tank healthy through Spirit of Runn Tum. Stay within the cleared platform area if the accompanying imps turn toward you.
Damage- Remove the imps while spell reflection is active on Pusillin. Weapon users can continue on the boss; spellcasters should avoid reflecting direct damage back onto themselves.