I would actually go with Law here. Even though he is strong, I believe he won't be able to take Caesar down, otherwise he wouldn't ask Luffy for his help, so for me the one who is going down here is Law, especially since Caesar has sleeping gas and poisonous gas...
First, let's be honest, given what we've seen until now, there is just no way Luffy will win this tournament ^_^
WB is a sure win here, now let's see how Luffy would compete against Law :-/
The problem here is that we know Law is pretty good with his ubber devil fruit and we've only seen Luffy fighting without being serious.
On one hand, we have law who is really fast in his room, master everything in it and calculate thing to get things the way he wants and on the other hand, Luffy just go on instinct...
Right now, I can't help but think that during Smoker Vs Law, you could switch Smoker with Luffy and the result would still be the same
Do you really think law has no chance against Luffy ?
I know he's the hero and stuff but Law appears to be a real Tacticians and his devil fruit his pretty strong while Luffy fight are almost only Luck Based (almost ^_^)
Imho it all comes down to speed : If Luffy is as fast or faster than Vergo, he could win, else I don't see how Luffy could compete against Law in his Room (once again, given what we've been shown until now on both of them and letting pure fanboyism aside )
Ps : regarding the mixed group, we are still on preliminary rounds, groups aren't mixed until two or one contender stay alive (as in football/soccer )
In this group I believe the ones who should win are:
1. Whitebeard. He is the obvious choice as he was seemingly the strongest out there after Roger's death.
2. Luffy. Of course people can argue with me, but his speed seems superior to Law's teleportation and the only thing he needs is to lay his punch as after this Law will have to maintain his counciousness and won't think that much about teleportation. As well Law wasn't shown as a resistent type fighter as he was taken down by Bergo both times with quite an eaze.