How I did that job:

First, I used a circular saw to cut smaller areas. To find the correct depth, I started by setting the saw to cut 1/2" deep and cut a small square about 6" square. If that did not come up, recut 1/4" deeper---and so on. I used cheap blades because the blade was going to hit metal fasteners. AND EYE PROTECTION!!!!!!!

Depending on how many layers and how many fasteners, I'd cut 4' squares or less. Once there were three layers of 1/2" ply---did the first layer(LOTS of staples) and then the other two.

I used flat bars and full sized crow bars. There are specialized floor removal tools(like roofing tools but heavier duty), but I never spent that much since I only did about one floor a month or less. I also had several short pieces of 2" by 4" framing lumber to use under the prying tools to increase range of motion.

Lots of work, no other easy hints.

If that buried sheet flooring is asbestos and you run a circular saw through it well enough said.

You might want to consider renting a floor stripper tool that will save you a lot of time and effort and probably do less collateral damage to the actual subfloor than chipping and prying away at the old stuff. Something like this maybe: