Hmm...
I'd have to take Karate Kid for a majority in this one. The Turtles teamwork might help them score a couple victories, but Karate Kid operates at a different level in matters of unarmed combat, and his speed is uncanny, even for the likes of Leonardo and Raphael. I'd say KK 6-7/10, possibly 8. With Splinter added to the team I'd give it an even split. As for Shredder, it depends on which incarnation of him you use, but likely a split as well.
This is complete and utter spite. KK for the easy win. He's just got WAY too much skills for the turtles. Splinter & Shredder maybe masters, but they aren't masters of "ALL" the martial arts like KK. I hope the turtles aren't counting on a victory pizza cuz all they're gettin' is a great big can of whoop ass!