Carbide bits work, a jig saw will work, and carbide drills, too. Or, use a carbide scoring tool and then knock it out. A small router like a RotoZip with a carbide bit will work, but don't rush it - it will get hot. A diamond tool works better. If using HardiBacker, a jig saw will cut it (and dull it). Watch out for the silica dust when cutting the stuff - it will get everywhere and is a known carcinogenic - cut it outside upwind.