how do I level PAINTED concrete floor?
Yeah, unfortunately, you're going to have to strip it up. 

The two ways I can think of would be to use a concrete grinding machine to chew up the surface. Just a light chewing/scarification will do. You should be able to rent a machine that will do it from your local tool "we-rent-it" type of place... or they would know who to talk to. By far, a machine will be the easiest to work with, even with all the dust. 

If you have to go the chemical route, muriatic acid (available anywhere pool chemicals are available) would work, but you have to be 100% sure that you clean up all of the acid when you're done... it makes a HORRIBLE mess and the fumes are a beast to deal with. I would choose the grinding machine any time before I would choose acid or a stripper. 

After either step, be sure that you wash the floor thoroughly to get any dust or residue up and then coat with two coats of the recommended same-brtand primer (if you use Custom SLC, use Custom's primer -- if you use Mapei's SLC, use Mapei's primer) before you apply the self-leveling cement.