1. Create a sprite
2. Name it Mask
3. Add "Erase" effect to it
4. Doodle in the sprite (a smiley or something)
5. Right click the Mask, Order, To Front (this will bring it in front of other sprites)
6. Tick "Force own texture" in layer properties
7. Move the Mask over other sprites and glee.
Nothing wrong with using layers, though. They cost nothing