Get that beam off me, hoverboy

Classifying these minions of the Monitor as 'personnel' is debatable, but they certainly act as autonomous entities, limited though they are. Sentinels come equipped with an irritating laser beam that really hurts. Luckily they're vulnerable to most weapons, though plasma weapons or the needler seem the most effective. The latter does a very nice job if you send in a stream of little pink friends, and the explosion might take out nearby Sentinels too. The needler also has the advantage of getting straight through any shield.

Sentinels can also be tagged with plasma grenades; and as with needle explosions, the blast can take out multiple minions in one go. Tag a Sentinel underneath and it gets blasted high up into the air, which is nice to watch.

Attacking with melee

Although it's not usually very practical, Sentinels take damage from melee. They seem quite resilient to frontal attacks, but if you strike from the rear you can actually destroy them with a single blow, even on Legendary. If you can't take the Sentinel by surprise, you can try to get around the rear by moving in very close and circling - potentially faster than he can turn to stay facing you.

Destroying Sentinels with melee is somewhat doing things the hard way, but it can provide an interesting challenge here and there.

Ramming them

Amusingly, you can also destroy Sentinels by ramming them with a Banshee. There's an opportunity for this at the end of Two Betrayals, where they're swarming outside.

Making enemies for fun

In the Library, Sentinels are supposedly your allies against the Flood. However, if you destroy enough of them in a group, the rest will take a dim view of it and start giving you the laser treatment!