I hope the Death Note works on them. If not, we better contact Aeon from Castlevania Judgement to bring help from the rest of the multiverse if we even want the slightest chance of having a chance at victory.
Chara is an extremely good manipulator and someone as powerful and sheltered as Flan would be more than useful for them to carry out their "purpose." On the other hand, Remilia can manipulate fate... not sure what good that could do. It'd make an interesting fanfic for sure.