The nut-in-a-pillar inside most 100-series PBs is made of brass, and it is embedded in a pillar of ABS plastic which is integral with the case top or bottom at each location. The externally-accessible case-retaining screws are not brass, but they are metal-thread rather than a coarse pitch for plastic. It takes only the slightest corrosion between the dissimilar metals to form a 'cement' between them.
If you exert too much force when you attempt to undo a case screw you risk turning the brass 'nut' within its ABS pillar, which immediately smashes the plastic pillar. End of utility of the pillar, and it is a pain to reconstruct one using Araldite or the like, if you ever find the fragments of pillar.
The approach most likely to produce successful deconstruction (as opposed to destruction) is as already suggested to you. Spray a little release-agent onto your Torx-8 screwdriver tip to form a drop on its tip. Then place the drop of WD-40 or RP7 into each recess in the case where you need to free a screw. Try to let the drop run around the screwhead and down the thread. Now, as Burl Ives used to sing, "Let the old Earth take a couple of whirls". Time is the essence to let capillarity and the low surface-tension of the solvent work their joint magics. (Pun intended if you like.) The solvent is volatile, so, if necessary, apply another drop half a day later, checking for loosening each time. Use moderate force with the driver, first clockwise and then anticlockwise, to check for loosening of the joint. There is no alternative, short of destroying the case, for getting the case apart.
Of course you have the main battery out of the case already. If not, you need to know that the battery also holds the case halves together. If you have not yet inspected the insides of the PBs, perhaps it is premature to be concerned about turning either of them on.
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