My personal suggestion is don't let the players get a break. have them face monsters who by themselves are challenging and thus lower their resources. The party caster will have fewer spells, thus making him more venerable, considering an 8th level caster has very few 4th level spell slots.

On top of that, I like the Dungeonbred template, as well as the Half-Illithid template (forgot what book tho...) to give your grells the extra scare-tactic-esq abilities.

In a final note, to limit caster cheese, use terrain to hinder the caster's spell list. A low ceiling and/or a rope bridge makes the battle not only cinematic, but hampers the casting of fly and wind wall, since he can only fly barely out of the range of Grells and his wind wall would rock the bridge, thus making his allies less capable of effectively fighting. Give the Grells some minions to keep the tank-y players at bay, such as Goblin Archers or Kobold Slingers, who while limited in the ability to hit, can keep the tanks occupied while the Grells focus on the squishy foes, such as mages and archers.

hope it helps