I had fun designing the boss for my campaigns first adventure, giving him spells that I knew would mess with each individual character. For dealing with the gnomish cleric I gave him Reduce Person, which he used, making the character Tiny.
Druid (who’s wearing the skin of an orc he killed like a disgusting, horrifying onsie): I’d like to cast Summon Nature’s Ally
DM (me): a wolf like usual?
Druid (really I can’t tell you enough how gross he was. He rolled pretty low of the skinning so the orcs hands were still inside the skin): mmmm… nah gimme a monkey
DM: your monkey appears in a flash of light
Gnome (who’s currently just over 1 foot tall): I RIDE IT INTO BATTLE
DM: You… ride it into battle
And I’ll be damned if that duo didn’t take down the sorcerer