Sersi vs. Impossible Man

Can Sersi be anyone's Kryptonite and stop one of the biggest marvel has to offer?

No

Unimind should be able to come up with some object that can provide like the oracles do in Olympus.

Take for instance how Sersi can change part of the moon into green cheese.
If she transmutes it for a limited time and it changes back to moon rocks it would be as deadly as the Invisible Women expanding bubbles in a person.
Now in that case if Impossibe Man doesn't become angry about haveing to pee kidney stones I'd think something was up.

That makes total sense.

And then when he gets so angry he turns into a hulk before he can transform she puts a shield around him so that his transformation turns him into a powerless human by denying him the gamma power.