I can see this going one of two ways...
In a long, even fight, Supergirl defeats Isis and, then, in a not so long and not so even fight, Wonder woman defeats Supergirl
... or...
Wonder Woman defeats Isis... AND Supergirl... at the same time !
Having only just now thought of it, the second of these versions is growing on me more and more...
The two lesser heroines teaming up to give Wonder Woman a good go... but, in the end, I can so visualize Wonder Woman, holding Isis under one arm and Supergirl under the other, in a pair of side-headlocks... simultaneous driving each woman down onto her knees...