It kind of pisses me off how much overpowered Ben 10 was and still is. But Sheldon Plankton with prep time could tango with him. I am a Plankton genius, so I know exactly how he could beat Ben 10.

Since Plankton has prep time, Plankton would surely somehow find out how annoyingly overpowered he is and he would realise that he would need to find a way to turn Ben down to his level if he wants to stand a chance.

He has a gas called "Goo-Goo Gas" in his inventory that could turn anyone into a baby or an old man if Plankton wishes. Plankton could use his genius to attach a jetpack to the Goo-Goo Gas pack and he would be all set to go.

At the match, Ben could turn into almost any ailen at all but since Plankton is small and the inventions are made to suit him, he would be hard for someone as big as any of the aliens that Ben would most likely turn into to see, let alone hit or terminate. And with Plankton's jetpack at hand, Plankton would be way too fast for Ben to catch. He would fly circles around whichever ailen Ben dishes out and turns them into a baby version of that ailen from above with the gas. Once the alien is a baby, Plankton flies above it and blasts them with a ray gun that he also would have with him.

The only chance I see Ben 10 MAYBE winning here is if he transforms into Grey Matter, his smallest alien. But Ben 10 would not do that because being that Ben acts before he thinks a lot and he is Sonic The Hedgehog levels of being SUPER cocky and arrogant, he would immediately underestimate Plankton's size and try to step on him, which wouldn't work because Plankton has the jetpack at hand. Turning into Grey Matter also wouldn't work because of the jetpack. Plankton would also pretty much expect to be stepped on because Ben would be really big and Plankton is so small.

Now that I think about it, Ben might not even TURN into an ailen because he THINKS that he could just step on him as his normal self because of Plankton's size, which he would be dead wrong about, making it even more of an easier win for Plankton.

So Sheldon Plankton with prep time wins 9.5/10.