The last screw will not loosen, it may be in crooked. It looks striped now. Why do they put the screws in so tight?
I'm gong to guess from your forum profile name that this is a vivofit 1 and also because this has user replaceable batteries.

If this is correct, I would try a new screwdriver that is sized correctly for this type of screw, especially if the screwdriver has lost its grip due to damaged screw head. If you could take a picture and post it here, that would help identify the issue.
Vivofit 4
Moved thread to the vivofit 4 forum.