Ben 10 does have his fire form, but it wont be enough to melt this icy opponent. Iceman has taken many heat based opponents down in the past and I don’t think that he will stop now. Iceman’s battle experience may have been the deciding factor. He has dealt with opponents like the Avengers in the past, so he knows how to handle himself. Iceman wins.

Update 10/25/2022 Ben 10’s forms should be able to even get past Iceman’s ice. His fire abilities and other forms would just melt any of Iceman’s attack options. Ben 10 wins.