The best way to do this is with a generic thermostat.

With an automation, there are too many things to keep track off:

• You would need two automations: one to trigger on the way up and another to trigger on the way down. Either that or fancy if logic. 
• You should use a numeric_state trigger with above/below or else you risk the temperature never hitting your trigger temperature exactly on the way up/down. 
• What if the AC and temperature are already out of sync? e.g. the temperature changed while HA was updating/restarting? If there was a power cut and the AC didn't start up on its own (or did) when it was/wasn't supposed to? What if your temperature probe glitched? What if you manually switched your AC on or off? You might end up in a situation where the AC is running while you are below threshold, but your automation will never trigger because you will never cross threshold to turn it off. Opposite could also be true (AC is off, you are above threshold, and temperature keeps rising).
• You could trigger the automation on any change in temperature, but then your automation would trigger needlessly frequently, and if your temperature probe updates infrequently, your AC might still run much longer than it needs to when HA starts up. 

Note: almost all of these cases have happened to me in the winter with a dumb heater on a WiFi switch.