I'll look into both the Bonzai Bill and the Pile of Bones issue shortly, but casting the spell was so difficult because there's no way to boost the gem cost without boosting the fatigue cost. I'm also beginning work on my next nation mod.
And to give you something to look forward to on the Bonzai Bill front, it's as fast as a Bullet Bill, size 4, and has trample.
EDIT/UPDATE: It seems I forgot to give Bonzai Bill a morale attribute, and screwed up his spr path. He also wasn't
nearly as tough as I thought he would be, so I decreased his research level to 7 and now he costs only 3 earth gems. His pathlevel also decreased to 4. This, along with fixing Dry Bones, marks the debut of Mushroom Kingdom v1.01