Frost bow is not a great counter to Undead, because you're just holding off the inevitable as they get stronger and stronger. (sounds strangely appropriate to fighting undead...) Falkow does have ways to deal with them, however. All the undead who revive off their back up copies are RNG 1, the Sky Staff is one option. Let them invest 2 or 3 SP pumping up that Hell Smoke, then return it and you've got a 6 SP advantage. Even better, since both Broke Priest and Madness Priestess are RNG greater than 1, you don't have to worry about accidentally returning them. Return their front line units, then beat down the back.
"Scissors are overpowered. Rock is fine." -Paper