So borrow it back each month
Either the 0% APR won't hold, or there'll be a 'balance transfer fee' or what ever it's called, of 2 - 3%
So a practical point, is that the entire balance won't be able to be borrowed at no cost for the entire 18 months.
Otherwise, the scheme has some merit. If you have cash/ funds available to pay it off along the way, you're getting an excellent rate for a loan. Also, be careful to have funds available to pay it all off at the end of 18 months if it becomes necessary. (not approved for the right cc....)