From Archon Arcana
Exterminate! Exterminate! • Mars • Action • Uncommon • Play: For each friendly Mars creature you control, destroy a non-Mars creature with lower power. • Artist: Marko Fiedler • Card Number: AoA (180)
If I play Exterminate! Exterminate! does the active player choose Mars creatures one by one and for each chosen creature destroy a lower power creature and remove it from the board before proceeding to remaining Mars creatures, or do we have to choose all creatures and destroy them at once? If the latter is the case, can more than one of my Mars creatures point to the same lower power creature?
You make all choices first, and then the chosen creatures are simultaneously tagged for destruction. You may not choose the same creature for destruction multiple times, so each Mars creature will have to point to a different lower power creature.