I used one of these, sometimes called a "johnson bar," it is basically a floor scraper but made of heavy steel and though it is exhausting to use after awhile, its heft allows you to get good momentum behind it, more effective at pulling the floor up than a small scraper. It is a good workout!
My steel floor was super rusty causing the adhesive to fail in some spots, so the rubber came up in sheets. The one advantage of having a rusty floor!