Hey
So your definition of cheese is hitting them " with a massive fireball"?
If ive got a 5th level wizard in my party im sure theyll use fireball and or glyph of warding as they are great spells. But fireball isnt required to win the fight.
Some players use rain and cold/ lightning damage.
Is pre-buffing cheese? You can use mirror image/ blur scrolls, potions (strength is great), shield of faith, the absolute shield, spells to buff hitpoints.
Positioning: take the high ground and blast the Gith away with thunderwave etc.
Have your summons ready, buffed and in position.
Keep plenty of healing potions and haste potions.
Clerics with spirit guardians is good.
Laezel doesnt have to run off to them either.
Summon the Ogres and use the little spiders.
Of course bless is a great spell (which can be enhanced by a certain item).
i dont bother.. but you can use the wyvern poison. I cant recommend barrellmancy...but some players us it.
Good luck with the fight!
