Last I remember, if the all the living opponents are in critical condition, they will still attack you. When there's only one left, and it's critical, that would count as all of them being critical, since he's the only one.

Turning the last enemy into a frog, on the other hand, works very well. In fact, turn the final enemy into a frog, then turn everyone else into frogs and have Cloud start attacking everyone. In case they have some sort of counter reaction, they'll do little damage with it . Using Beowulf to keep the last enemy a chicken would help a bit too.