We have used a power scraper you can get from a rental company. its a bit dusty but will do the job. you could also use a self leveling underlayment to cover the whole floor. this method would give you a smooth surface to install your new floor on. that product will cover 12sq/ft at 3/8in, it hardens to a walkable surface in 3hours. we use it to top our floor heating systems prior to tiling.