The Comic Book of Spells: An Unexpected Sorcery: Brand's Thunderclap

Thunderclap
Brand stepped through the portal and approached the slime, flipping his spellbook open. A homemade collage of comic panels isn't a traditional spellbook by any metric, but if comics had taught him anything about magic, it was that you had to believe in it for it to happen. 
He flipped to the page with a hero calling down lightning from the sky, staring at the sound effects paired with it. Raising his hand, he stared down the gelatinous creature and recited the onomatopoeia as though it were an incantation:
"KRA-KOOM!" 
He felt something leave his hands, but it wasn't lightning. Raw force and sound blasted from his palm, as mighty thunder shook the air between them. The slime rippled and shook from the sound, causing waves to shoot and shake all over it. 

Brand was confused by this, looking back down at his spellbook. He was going to have to figure this out later, but for now he needed to focus on the battle. If the lightning spell wouldn't work, he'd just have to try something else. Dodging a spew of acidic fluids from the slime, he flipped through, searching for another spell, anything that could possibly save him from this predicament. What were slimes weak to? Not bludgeoning damage, not poison... maybe water? It depends what the slime in question is made of... 

Forget it! Brand ducked under another volley of spit. He was committed already, and struck again with his magic.

"KRA-KOOM!"

Another wave of sounds, hit.

"KRA-KOOM!" 

The waves traveled over it, forcing it into unnatural shapes, until...

The side of the slime ruptured, spilling its fluid onto the ground.

Not an elegant win by any means, but... Brand survived.