Thinset is not simply tile glue; it is a cement used for adhering tiles to walls or floors. Many chemical adhesive removers will not soften thinset like they do mastic. When thinset has hardened, it is very hard to remove. You may not have to use power tools, depending on the age and brand of the thinset and condition of the subfloor. Hand tools and a bit of work should accomplish it.
How to Remove Thinset From Concrete Floor
Put on an N95 respirator (or, at the least, a common dust mask) and safety glasses. Leather gloves can prevent blisters.
Starting at one corner of the concrete, use a razor scraper (a long-handled one if you are working on a concrete floor versus a wall) to chip away some of the thinset at the edge. If it is coming up, continue scraping it.
Visit a home improvement store and ask about renting an electric floor chipper, grinder or similar tool for removing thinset. This might be necessary if you are unable to get the thinset up with a scraper. These are usually simple to use; you walk behind the machine, holding onto the handle to guide its path.
Finish removing the thinset, either by scraping manually or using the power floor scraper/grinder.
Sweep the thinset dust and debris from the flooring with a stiff broom. Do this before you put away the tools; you might have missed a spot and not noticed when the floor was dirty.