If your screw or bolt is already free and you're just trying to give it some "tooth", make sure it is secured by a vice or clamp. The important thing to note is not to use your hands to hold it! It will get very hot, not to mention if it catches on the cutoff wheel, you could be injured.
Fasten a fiber-reinforced cutoff wheel onto your small rotary hand tool. In the past, I have used a silicon-carbide disc that did not have fiber reinforcement. It worked okay, but on the smallest catch, those wheels will shatter. It's best to use one with fiber reinforcement.
Now cut a notch dead-center in the screw/bolt head. Take your time with this part. It could be your last chance in removing the screw or bolt, so make sure you get a clean, clearly-defined cut. Go deep enough to give your screwdriver some tooth, but not so deep that your screw or bolt becomes weakened to the point of breaking.
Once it is cut, use a good flat head screwdriver to remove the screw or bolt. You can also give your screwdriver a tap with a hammer to really sink it in before applying torque.
I hope this helps you the next time you have a stripped screw; it has saved me a few times.
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