Many mastics soften when wetted. Is the area someplace where you could get it all wet and let it sit for a bit? If so, it will likely come up fairly easily with a scraper. A steamer might work pretty well, too. You can get scraper blades that fit on a pole. Buy a box of the blades, as they get dull fairly quickly over a concrete surface.
If you're planning to tile the area afterwards, do NOT use a chemical stripper. In fact, the mastic may have seeped into the pores of the cement, and the only way to get a reliable thinset bond would be to run a scarcifier over it to remove a thin layer. That will also rough it up a bit, also improving the thinset bond.
The way to tell if thinset will bond to the surface is to spread some water on it. If it beads up, thinset won't bond well. If it gets absorbed in less than about 15-minutes, you should be okay.