Yeah the Anti-Gravity thing could cut him but then Goop would have to follow it sooo.. big disadvantage there. Assuming that it's even strong enough to cut through Swampfire.
Prettyyyyy sure Goop can't blast high radiation acid, it's just.. acid.. but yeah I'm sure that would melt Swampfire. But then again it has been shown in the same episode where Swampfire freed himself that he was able to regenerate from mush which if you ask me already makes Swampfire A LOT harder to beat.
We never see how Swampfire's fire would react with Goop's acid so there it could go both ways.
But even so, all that Swampfire has to do is turn off Goop's artificial gravity to win while Goop has to somehow beat Swampfire into less than mush to win.