I want to clear out a mountain to start building in it but its been taking forever to use a pickaxe to clear it so I thought if it was a really easy way to get tnt that would be faster.
Honestly if I needed TNT I would just get into a desert at night and kill the creepers with looting sword... it's easier to see at night in the desert. You could always build a mob spawner or have you thought about just building the mountain?
The best option would be to use a mob grinder, and have a mechanism to sort out creepers. I don't know how to make an effective one, so you"ll need to either look it up, or ask someone else on the forums.
Creative?
May I ask why you need "a lot" of tnt? Depending on what you're trying to do, there may be a better way.
Sorry that I'm not much help