The Grundle is the 18th level of The Salt Factory. A conveyor is moving you to the right. Two Sawblade Launchers are constantly shooting at the same time, both move in opposite directions. At the right end of this level, there is a key. Take it, and return to the beginning to reach Bandage Girl.
To get the bandage, you would do the level all the way, but before you get to the end, there is a hole above the end, if you climb to the top of the building and fall through the tiny hole in which the bandage was placed, you would get the bandage. Keep in mind you have to beat the level to keep the bandage.