If the screws are completely stripped this probably won't help but I used to have the same problem until I started using WD-40 on all the screws in the lower housing before trying to open 3DS consoles up. I spray a bit into a plastic cup, dip the tip of a straw into the cup and drop a tiny bit into each screw hole. After leaving it to sit for about an hour the screws are much easier to remove, though they still require quite a lot of force.