I making a "Door-Clearer." If there's a creeper at my front door, I press a button and a piston releases water that pushes him into a deep shaft off to the side. :D???
It sounds like it'd take too long
Plus the creeper wouldn't be dead
I suggest digging out a huge 1x1 or 1x2 space (depending on how many doors you have) and making it deep, then filling the bottom with lava or water. Then having sticky pistons either push/pull the blocks above the hole.
With water though, it needs to be a deep enough fall that it would die. The water could act like a mob trap and bring the loot to a holding chamber underground.