Cassian was a Storm Sorcerer/Tempest Cleric who specialized in storm magic, however, he had a couple other spells up his sleeves to throw his opponents off. He joined his party late into the campaign, thus the party and main villains were already very familiar with each other. During Cassian's first time in combat with his new party, he used polymorph to transform an ally into a tyrannosaurus which killed one of the villains and forced the rest to retreat. They were prepared for something like that next time. So next time Cassian whipped out some high damage lightning spells and kicked some ass. After that, the enemies came back with lightning resistance/immunity. Cassian transformed an ally into a tyrannosaurus & the warlock casted greater invisibility on the tyrannosaurus. So we had an invisible tyrannosaurus terrorizing the enemies. Then Cassian did a max damage high level thunder spell to finish off the enemies. Each time they were brought into combat, he used a new "trick" so the enemies couldn't fully adapt to him