I repaired my Ultron 35mm a year ago, and one of the internal screws did not come out, just like you are describing. I ended up using a dremmel tool to cut the head in half, so that I could grip it with pliers and forcing it out that way. I put several ugly marks on an interior surface of my lens. You can't see them from the outside, but I know they are there.
Now I know a better way. First get exactly the right screwdriver. There are many different philips head parameters, such as angle of the hole and depth, as well as the size of the cross. They make different drivers for each of them. Then assure yourself that it WILL come out. Apply slow, steady pressure using the proper driver. Do not try to "rock it" out, as that will begin the process of stripping the cross, which will just make it worse. If you suspect that someone has used LockTite (which is a BAD THING!) then you can break the bond by using a small tipped soldering iron to heat the bolt. The LockTite will break at a relative low temperature, about 250 degrees or so, easily obtainable with a solderer.
When you are reassembling, use a drop of enamel paint to lock the screw in place, not LockTite, or epoxy.