Okay here in the first pic you can see two screws, the rounded head phillips and one I just have for example as I mangled mine pretty good getting it out.
What you want to do is push with the screwdriver/small chisel against a "face" you're going to carve in the head of the screw. Imagine the cross of the screw slots dividing the head into four slices of pie. You carve out a slice with the chisel so you can push against that face to loosen the screw.
Couple different pics of the same angle to use to lop off a piece of the screw.
Just because your screw's shape may be altered already, you may need to nake the chisel and going straight down, what it a couple of times to reform it somewhat. And you might not get it right the first time. Which is why mine looks like I carved an L out of it.
One more thing, don't try to do this with it sitting on the ground or bench. Do yourself a favor and put it in a vice. Hope this helps.
Oh, and I don't do disclaimers.