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.
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.
If you use a power tool for this, there will be a lot of dust, so keep several N95 dust masks around. The filter will get clogged after a while. Wear a head covering if you want to avoid getting thinset dust in your hair. If the layer of thinset is very thin, you can also try simply sanding it down (with a hand-held power sander).
If using a power tool for this job, keep the cords safely out of the way of moving parts.