First, I gonna assume that the source of the water problem is fixed. If you haven't done that, it's not worth fixing the stain problem until its source has been eliminated.
Assuming the source of the water has been eliminated, make sure the area(s) in question are dry. Remove any loose drywall and paper. If there's frayed paper, use a utility knife to cut off only the loose parts. DON'T just rip it off. Once this is done, get some joint compound (also known as 'wall mud'.) If you don't have any bad water damage that requires replacing anysections of drywall, go ahead and put on a rough layer, or 2, of joint compound. (You'll need a spackle knife if you're doing this.) Rough-sand between each dry coat the next day. Your final coat should be smooth and you can sand it level. If it's a sunny day, natural lite is the best way to see any uneveness.
The areas you had to patch and the stained areas now need to be primed. Go to any home center and buy some oil-based / alkelyd primer. Stain Kilz is a popular one. Get an oil-based paint brush too. Wait for a day you can open windows because this stuff is noxious! Invite a friend if necessary over to hang around while your condo door is open (with the security chain on of course). You definitely want cross-ventilation. Since you're working in a somewhat public arena, put a fan in the window to draw air from your hallway or another room. Air out = Air in. If you're talking to your neighbors, warn them as a courtesy. Put a note on the door too as a courtesy.
Since the Stain Kilz is oil-based, it will seal the area giving you problems. In short, you need to prime before you paint (you need to seal before you paint). Since you've been using a latex paint, the water stain has been 'bleeding' through your paint. That's why you still see it. Priming is a general practice when you want to paint. You should tell your paint dealer what you're trying to paint because they'll know when you need to prime. (Example: You need to prime if you have things like water stains and fresh joint compound. Both are what you (may) have.) After you prime, wait a day. Then paint. You should expect to put on a 2nd coat of paint (not primer) to get a real nice looking wall. Be sure to get the right rollers, brushes, and spend a few extra $$$ and get a good quality paint. Finnaren & Haley Paint, Pennsylvania Dutch Boy, Benjamin Moore, MAB, and Sherwin-Williams are a few. My favorite is F&H. You'll pay $20-30 a gallon but you'll love the look and you won't be repainting in 3 years. Stores like these sell their brand of stain killer too. Stain Kilz is a brand name.