Can't attack Bone Church in Dark Priests

I have FOUR generals on the island in this adventure,
and every time I try to do a Battle Preview on the
Bone Church it give me some msg about
"It's possible to destroy with one wave, but it
contains a boss enemy so two waves would be better."

Same with Dark Castle.

Can't you do a Preview?
Is this just the advise the Preview gives you?

the battle 'preview' is pretty useless as its been implemented. just check a guide for the best attack on them. i have a vet, so i myself use 48R 1ES 100C 101XB on the bone church, and 83R 1S/ES 17B 149XB on the dark castle