I am going to attempt this, but instead of the entire CMC vs South Park, I’ll just pick one, if that is ok.  
Scootaloo vs. Cartman  
Begin!
 
“Whoa there, look at what we got here  
Some fat boy trying to be the devil’s rear
 
Your mom’s more of a whore than Button’s, just trust me  
And your video game had way less control than Bubsy
 
You think killing parents is the thing  
Well guess what, one pony with some wings  
can relate to that  
considering Scott didn’t deserve to be treated like a rat

@Shademeister  
You forgot to factor in the sheer chaos the CMC create in their wake. They’d probably kill Kenny by accident.

@PLCTheCd  
as long as Kenny dies in the second verse i don’t care who rights it

Anyone else thought coming up with the whole lyrics?

Nobody wins, everyone loses…

@Spiritus Arcane
 
Well, Seeing as how the South Park boys have actually killed people and the CmC are just tiny horses who don’t know their spot in life. I’d say the CmC’s are in the WRONG neighborhood.
 
Also, I kinda wanna hear this. It could be funny, especially for Kenny’s lines… It just starts out with “Mmmph mphm mmph mph mph” then he just unties his hoodie and unleashes the beast of Mythos upon them. You know… Since he’s Hastur and all.
 
Kenny: “The name’s Kenny McCormick and this ain’t no limerick. Bout to drop these ponies straight into a panic. You’re a bit outta place, cause if you ain’ food… Then its off to get your cutiemark!… As wood glue. I’ve died a million times, come back for more! Had more lighting inside of me than the hammer of THOR!…” ect ect.

yep, they’re doomed.
 
no, not them. not them either.
 
no I mean the audience.

Screw the rap battle, I just want to see the CMC vs. those four. I honestly have no idea who would win.