Help with Events and Layers

[Zant]

I have 6 mother brains in six different layers, attached to 6 friendly parakoopas(which are all 6 in a singe layer).
I want after the mother brains are all defeated, event x is triggered.
But, the mother brains are each in a different layer(for parakoopas to work), and I can't seem to figure out how to make event x trigger when all mbs are dead.
Any ideas?