Id have to go with the team, Plastic man and Atom.... Plastic is nearly immune to physical attacks and energy attacks, and since his got comple molecular control from his body, it would be near impossible for sersi to use her more exotic abilities Ie. subatomic manipulation. This would even be a fair fight without Atom. I dont know much about Atom, all i know that he can shrunk and quite intell.
Id give the team 8/10 wins.
Plastic Man and Atom would generally lose, but they've got these occasional absurd showings that might allow them to win. The Atom has gotten so small that he's a theoritical particle, which made him immune to magic -- I'd imagine it would do the same to Sersi's cosmically related powers.
Plastic Man is indestructible and has faced down the DC Circe without giving in. But stll, I'd say Sersi wins more often than not.