FF might actually win, but they don't win easy battle and they have to fight smart and defeating Ivy without prep is not easy task for them (even with Reed Richard's tactical and scientific genius)...
Medusa and Clayface aren't big factors there (although I can see Clayface keeping Thing busy for a while, but Medusa's malleable hair is a bit laughable). Reed doesn't have to participate in the fight- he may coordinate the team's action while being protected by his wife's forcefield.
Then... Cannonball (I feel tempted to write Cannonballs) vs Torch- battle of two characrets I don't like, but Sam has more possibilities to KO Jonny than vise versa...
As for complexbrother's setting... I agree on Clayface and Medusa being beaten by Torch and Reed, but...Thing has to catch Cannonball first and he's not fast enough for doing that (unlike Colossus, who's very fast and agile for a brick).
Separating Ivy's body won't work, because Sue's forcefields are unable to isolate her mind from contact with the Earth's biosphere...
Ivy and Cannonball are the last standing ones for their teams, and Ivy has everything to win this. Unless prep is involved and Reed invents some plot device gadget to isolate Ivy's mind from contact fith biosphere like Lutor(if i'm not mistaken) did to Swamp Thing. I'm not sure whether Sue's forcefield would be enough for that...