I guess I would keep going at it after letting it soak for awhile, the threads could be rusted or even bad. Try not to let it slip, but if you do end up stripping it completely out, you can take them out with an easy out or a reversed drill bit.
Like I said earlier, I am not familiar with the flatsides, but from Darkstar's post they sound basically the same as a round slide. No hidden tricks or anything.
BTW - Now that I think about it, I have taken apart a roundslide that only had one screw holding it on. I never had a problem with it leaking after reassembly.
*edit* - I have to head to class, good luck.