MTGNexus

Was going to say polymorph but that didn't seem accurate looking for cards that weaken creatures/turn them into something else. Like I destroy that buuut you get this.

So far: Looking for similar cards in her colors. Or if you have a better idea for the commander I'm all ears.

Are you considering only spells, or are creature abilities also an option? My favorite old school version of the effect is Ovinomancer - which when paired with a haste effect (I like Thousand-Year Elixir) allows you to cast then tap for a sheep in response to his ETB trigger returning him to hand instead of paying the land-bounce. I once used this with High Tide to "Ovinize" 9 Slivers in one turn. . . good times.

PS: You may want to edit your post and link your possible commanders, so players that may not yet have them completely memorized have a reference in-thread.

Have you considered Phelddagrif? Bant has the most "You-Can't-Have-That-Have-This-Instead" removal and there's an excellent primer on this site that uses such a unique removal suite to control the board while playing politics to stay alive.

Some notable cards include, but are not limited to:
It's a little less specific than going full Transmogrification Station, but it should also attract way less hate than Hinata. Hinata just screams "I will combo so kill me" even if that's not the plan.

*gets out scales and a duck*

Kasmina's Transmutation
Mass Diminish
Humble

Stroke of Midnight and Contraband Livestock mentioned above are perfect.

Mirrorweave for maximum lol's

Use Asinine Antics!

Witness Protection
Metamorphic Alteration
Polymorphist's Jest
Turn to Frog
Crib Swap

Those are some more that might fit your theme.

Transmogrifying Wand is even 3 uses!