possible, but iirc there are some flying enemies which = more units to deploy (and I think there are witches), and there isn't such a nice chokepoint where you can catch all of them except the spot right before the heart, but you need to place archers to kill the flying enemies which would mess with betty's kills. not to mention the drops will be halved as compared to 2 runs of pastoral gate (charisma cost is almost double as well).
so...all in all, mysterious beast might work, but it's not as good as pastoral gate. (mysterious beast was longer too, and I remember having to wait a while before some of the waves started) too lazy to check the exact values though.