The other thing I've been doing is working on the level format:
Now I can pass as many background images to the background object as I want, and it will string them together into a single level. Enemies don't activate until they appear on screen. Of course, now I have to split the level into sections, with the player having to beat a certain amount of enemies before progressing. This also means making sure enemies don't go to an area the player has passed in order to search for objects to throw.