I'm not familiar with you particular phone, but I am with small screws...and it sounds like they, or the holes they're in, are partially stripped.
Most times you just need a little side pressure on the screw head while turning the head. This provides a little more grip for the threads. I'd suggest a sewing needle. Try to get the point under the edge of the head, and apply a little bit of upward pressure while turning the head.
If the screws are recessed, you have more of a challenge. I usually try to very gently pry the case apart (a tiny bit) to provide some pressure on the screw. Pry gently because you could crack the case. The blade of a small knife can do the job...it just needs to be very thin to get into the crevice. You might have to only partially unscrew each one a turn or two and work you way around the case. Just be patient and don't force anything...plastics can be brittle.
Once the screws are out, you need to consider what to do when you reassemble. You don't want to simply put the screws back in, since you'll be faced with the same issue next time. So, get a small tube of Loctite...the blue version. It's a temporary thread locker. It holds great, but you can still remove the screws next time.
Hope this helps.
