How should I remove tiles and backer board from my bathroom?
If you are removing the backer board, protect the tub (tape plastic over it and perhaps also use something like moving blankets or cardboard to pad it against impact) and use a crowbar and/or sledgehammer (with care not to go through the other side of the wall nor to damage the tub) to gain access for the crowbar to pry the backer-board off the studs. Or break it next to the studs, and then break what's left off the screws (if it was screwed in place), and then locking pliers to get the screws out.

If the backer board is not defective, and you are not removing the wall itself, consider the same sort of tub protection and using a wide, flat chisel to break off the stubborn tiles, followed by a "rubbing brick" to clean off the remaining thinset.