Sai and Kakashi
Deidara and Sasori
No Prep Time
Setting Hidden Leaf Village
I think the battle starts off as Sai and Kakashi vs Deidara in an aerial battle that the two of them might win, but Kakshi is going to lose a lot of chakra having to Kamui explosions and probably a few misses on Deidara. It's going to be made even more difficult by the fact that Deidara has encountered Kamui before so it's not a surprise attack anymore. Kakashi is going to have to strategically weave it into the gameplan.
So Deidara eventually gets beaten and you Sai who's a little bruised up from the explosions he's had to dodge and weave around as well as a Kakashi in similar physical condition but with maybe half his chakra spent. They're going to have to fight Sasori in perfect condition. Sasori has long range attacks that go far beyond any range Sai and Kakashi could fight from (laser water that extends to cloud level... which is like a few thousand feet below 30k feet). Their best bet is to have Sai stay in the air and hopefully get his dodge game right while Kakashi attacks from the ground. Unlike the previous fight, they can't get touched even once or they're on a countdown to death. Maybe Kakashi could do this as he fought against Hidan and was able to maintain, but that's a big maybe considering he had expert teammate Shikamaru and additionally he wasn't outnumbered 50:1. I doubt Sai could. He's never done anything impressive in combat outside of one filler fight. Oh, that's not true, he dive bombed an Edo, but in this fight he doesn't want to jump in the thick of 50 puppets and try to fight his way out.
Sasori prevails imo. The puppets will either get Sai or force him to be so far away that he's a zero factor and a weakened Kakashi can't Kamui his way out of everything Sasori is going to throw at him. It's just too much.