This type of Guardian was once a powerful beast, similar to the Stalker seen before. It has been mostly destroyed, with no working legs and, for the most part, is submerged under ground. They can still be as deadly as the Stalkers though, however their health pool is not nearly as high.
The attacks are the same, they still use the trademarked laser beam, which should not be taken lightly.
Like Beamos that have appeared in modern Zelda games including Skyward Sword, Link is able to defeat this creature by shooting its eye with the use of a Bow and Arrow. Ancient Arrows in particular are very powerful against these enemies, since one successful shot to the eye will take it down instantly. The Master Sword is also a wise choice, since its damage is elevated to 60 when in the presence of a Guardian.
Other strategies that can be used to defeat Guardians include utilizing the Perfect Guard ability, where Link can deflect the Guardian's laser with his shield at the right moment to take down the Decayed Guardian upon one successful deflect. Link should be wary though, since if he fails to Perfect Guard, the laser may still hit him and cause significant damage. The laser may also hit his shield, which often causes it to break with a single shot.