It sounds like it is either one of those pentalobe screws near the charging port, or an internal pentalope/phillips screw which is bound to be tiny. For both of those a lot of the traditional screw removal techniques won't be applicable. Here are some techniques I've used before that might be useful in this situation, and I'll provide some links to an ifixit page that I found.
- Putting something in the hole:
- Use something that is easily removable that you can jam into place with your screwdriver to help fill the gaps.
- I've used bits of small rubber band or plastic to do this, it often works if the screw is loose enough.
- Tutorial
- Using superglue and a sacrificial screwdriver.
- Just put a small dot of superglue on the screwhead (less is more
here) and quickly stick a screwdriver into the hole.
- Press it in firmly and try and get it to catch on something as best you can.
- Then let it dry. Some superglue dries faster than others, check your
particular container to see how long you should wait.
- Probably wait a little longer just to be safe, and then, while pressing down firmly, try to unscrew it.
- Tutorial
- This is more of a last-resort method as it may further mess up the screw (or at least leave it very messy), possibly get your device all glued up if you use too much, and might ruin your screwdriver (depending on the type of glue/screwdriver).
For screws this size, these are probably your two best methods for removing those screws. You could try a dremel to cut a slice in it but I would only try that with such a small screw if you don't mind cutting into the device or hitting other parts of it accidentally, as the screwhead will be so small that the radius of the cutting disc likely won't be able to cut into it without hitting adjacent parts of the phone. I wouldn't risk it.