There are a number of ways to remove stripped screws. Here's a page with what they describe as the 12 best ways to remove a stripped screw. A lot of those methods involve brute force because they're meant for larger screws, and I would suggest you shy away from things that tell you to use a Dremel or drill, since you don't want to risk damaging the bottom of your phone.
One of their methods involves JB Weld; you could attempt the same thing with super glue. Take a small screwdriver you don't mind never using again, place it on the screw, apply a very small amount of super glue, wait, and then try to remove it. Once again, the prohibitively small screws on the bottom of the iPhone become an issue. Another method listed on that page is using the largest flat head screwdriver you have that fits, and that has occasionally worked for me. We sell a precision screw extractor set, which may be your best bet.
Four different sized extractor tools to help remove fasteners or screws with stripped heads. Driver head is claw shaped for maximum grip. Sizes range from 1.1mm to 2.0mm. — 50+ available at 19.95 each.