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.

Head-butting and whacking

Amusingly, you can also destroy Sentinels by head-butting them with a Banshee, i.e. flying straight into them. There's an opportunity for this at the end of Two Betrayals, where they're swarming outside. You can even destroy a Sentinel with melee attacks. Not terribly practical I admit, but it could provide an interesting challenge.

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!