I simply find the spot weld and grind it. You use the edge, just like you would use a thinner cut-off wheel. I use the same tool to grind welds as well.
So find your spot weld and then use the wheel, grind until you start seeing the metal turn blue. When it does that you know that the top panel is getting very thin in that spot. Do the same procedure for a few more spot welds.
Then I take a thin chisel, actually an old KD-Tools gasket scraper and put it between the panels at the spot weld I want to break. Start gently hammering the chisel manipulating it until the spot weld separates. Sometimes you may need to go back and grind a little more. Better to be conservative in order to not dig into the bottom panel.
Any thin gasket scraper works well for this. Keep the angle edge facing away from the panel you are removing.