The Contra Brothers–decent mercenaries, TERRIBLE planners.

“Dispellers” is a blanket term that’s used to refer to a lot of different types of weapons (in their case specifically, special clips of ammunition that have bullets with dispelling enchantments mixed in with ordinary bullets).  These weapons are enchanted in such a way as to break through various types of magical enchantments and wards.  The effect that this has on fiends is that it severs the tenuous connection that their soul has with the astral plane, which is what allows them to reanimate (thanks to their Wolverine-like healing factor) after taking damage that would typically kill them.  If they aren’t dispelled, a fiend can’t be killed, no matter how much damage they take.  Serge carries especially powerful dispeller bullets for his Lucky Seven revolver, and high-enough level mages can replicate the effect with a properly-focused and empowered “Dispel Magic” spell.

So…yeah.  The Contra Brothers were not well-prepared for this situation.