try usong the allenk ey where you hold on to the long part and then try,it gives you more torque, which makes it easier to move, and if that doent work, maybe you screw is stripped, if th wrench is moving, but the screw isnt, it is most likely stripped
Nevermind the dremel if you don't already have one.
just use a hacksaw, and be VERY CAREFUL. Start by sawing backwards to start the groove (teeth should be pointing forward), then follow up with forward-backward motion once the groove is deep enough that you will slip too easily.
If you don't have that either, then just go buy yourself a sawblade at the hardware store. wrap an old towel, sock, t-shirt or just a rag around one end to use as a handle and Viola!!!
make user ur using the right kind of blade. Jewler's saws are a great tool. they may look flimsy but Ive cut steel pipes with them for metal shop. ^^" (forgot pipe cutters) to make a notch in a screw it should only take you 1-2 minutes.