What is going to be done to the floor after the mastic is removed?
If it is going to be polished or stained DO NOT use the Diamabrush mastic tool. It will leave scratches behind that will show in the polish or stain. If another floor is going over it then the Diamabrush is a good option and can be rented at Home Depot. Throw some 30 grit sand on the floor to help.
If you are polishing or staining after then removing it chemically is faster than removing scratches from the Diamabrush. Bean-e-doo works well for this. Spread it out and let it dwell for 45 minutes and it scrapes of easily.