Welcome to the forum bjcomix!
Placing a little penetrating oil along the case/case back seam and letting it sit for a day will usually do the trick. If it’s still stubborn reapply the penetrating oil and tap the case back with a piece of hard wood occasionally. To give you a good slip free grip you may need to use something like a rubber dish washing glove to grab the back. I’ve also found at times just twisting gently rather than trying to force it will loosen it right up. I have no idea why that works but can’t argue with success. Also for penetrating oil I use Automatic transmission fluid. You can also use brands like Marvel Mystery Oil, etc. I keep a plastic squeeze bottle around with ATF cut with some acetone (thins it even more) and use it often for stubborn clock and watch screws and locked up tools.
Hope this works for you bjcomix,