Using the `Guardian` effectively is quite difficult now that they aren't new to players any more. It's on par with the `Sentry Gun` from previous editions in terms of toughness. Your best bet is to deploy them around the corner from enemy spawn locations. Another good strategy is to have them backed into a corner so that nobody can get them from behind. 

In terms of counteracting a Guardian, this is relatively simple. To find them you can either use the `Engineer` perk, which is particularly helpful as you can see them through walls and floors and will have a very obvious red shading and outline, great thing about this perk is that it also highlights `Sentry Guns, Claymores and Bouncing Bettys`. You can also hear them when you are close because they make a sound as if though they are emitting radiation, but this isn't very reliable amid the noise of combat. Perhaps in `Search and Destroy` this would be a good method.

In terms of destroying them, you can shoot them (on hardcore, this only takes a couple of clips), use a `Black Hat` tactical, which is a pretty nifty tool couple with the `Engineer` perk. It will hack the enemies equipment and destroy it (Sentry Gun or Guardian) or, with Claymores and Bouncing Bettys, make them yours. Lastly, you can use a `Flashbang` and temporarily stun it, and you can use this time to run behind it and safely shoot it or get past it.