This is in the Barheim Passage after escaping from the Nalbina Fortress, right? If so, just kill them. When you kill them, any electricity they absorbed will be released. If the electricty gets low and the lights go out, other monsters will spawn, so try and kill any Mimics near you as quickly as possible.
To make things easier, I'd recommend buying your guys the best sword/dagger from the merchant guy near the save crystal. I don't really Fran and Balthier's weapons at that point (the bow and the gun), as swords/daggers make them much more destructive.
Just keep working your way through the area. I think at one point you need to go through a passage to the right and activate a switch to open a gate on the main path.