Gregor was keeping up his steady pressure, alternating between blasting Trainwreck and blasting Mush with one hand and aiming at Skidmark with the other.  Skidmark used his power to push away the worst of the slime, but it was clear he was losing.  His power wasn’t strong, it didn’t have much more push to it than a strong wind.  Any attempt to get it as effective as it had been at the edge of the arena took time and multiple layers of the effect.  In short, Gregor could make the slime more easily than Skidmark could get rid of it.