How do I defeat Sentrobes?

Solution 1:

Shield bashing the missiles will reflect them back, causing damage. It takes two reflected missiles to defeat one of these.

Another solution courtesy of IGN's Wiki guide:

Wait for it to fire a missle at you, and then use a shield bash to bounce the missile into its "eye." It will send out 2 "Sentrobe Bombs." Slash both of them across their respective blue line, with a horizontal or a vertical slash. If not, they will explode. The Sentrobe will fire a missile again. Bounce it back with another shield bash, and it will be defeated.

Solution 2:

Another strategy is to target lock and use your sword to defect it back. This is more fun, it's like fighting Ganon in OoT back in the day.

Solution 3:

It seems you can also just hit the timeshift stone in the room 2 more times if the sentrobe is positioned right. As the room shifts back to the present, the sentrobe will become a mechanical husk and drop, quite possibly onto sand. When it shifts back to the past, much of the sand will be replaced with yawning abysses, and the husk will fall into those before the sentrobe activates.