Add a few inches of water and a few pounds of dry ice. Lower an oxygen detector into the tank and when the oxygen levels are down to nothing, start cutting, if the detector detects any oxygen (say a breeze blows some in), stop and wait until the levels drop again before cutting. Any cutting can generate sparks, air chisels, torches sawzalls, what is important is to keep the inside of the tank oxygen free. Also it's best not to stand near the ends of the tank - if it was to go, the ends blow off and out (so they tell me, I've never seen it happen)
We've cut underground gas tanks in half for disposal using both the torch and sawzall that sill had heavy fumes.