A trap waiting in plain sight. But you've got the right idea. The material and magical energy to start the spell are already there; you cause it to wilt the mistletoe on demand.

[he has the right idea. he's definitely not... completely stupid. he could be catching on faster than shirou would, though if he wasn't, he still proposes ideas and concerns that follow logically.]

As for the borrowing... Certainly there are other types. They're much more involved.