The Guardian is a more complex class than it may seem at face value. If you’re used to older games, straight-up classic fighters like the Guardian generally mean that they’ll be easy to play. Not so with the Guardian, which has a dizzying array of combat abilities to choose from. They can effectively fight as tanks, damage dealers and melee crowd controllers to a certain extent, depending on how they’re specialized and how the player elects to use their abilities.
In most parties in PvE, a Guardian’s chief task will be to hold the attention of monsters to prevent them from getting to the lighter armored party members. They can do this through their various threat-inducing abiltiies like Provoke, which is particularly useful when the Guardian attacks multiple enemies at once, along with the careful use of their disabling combos. Their Counterstrike line of abilities is particularly useful for keeping the attention of single computer controlled enemies focused on them.
In PvP, they make for great damage soaks. Few people will attempt to target them first - which is often a mistake, because they can then spend their life harassing and damaging lightly armored targets, using their stamina burn combos and abilities. Abilities like Stormstrike from Tier 9 in the Tempest tree can be fearsome indeed, dealing lightning damage and flinging their target a distance away from them.
In many cases, the knockbacks that Guardians have should be used sparingly. They can be useful for limiting the amount of damage done ot the Guardian, but if there are many other melee damage dealers, it can lead to an interrupt in the flow of damage. Guardians work best when supported by a line of ranged damagers, in part because their crowd control abilities rely so heavily on knockbacks, staggers and other similar abilities.
The Guardian should always try to make sure that they have all of their relevant self-buffs active before they enter an encounter. They’re a major part of the class, and without having them active, they are much less desirable. It’s wise, for example, to pick a stance and stick to it - either defensive or frenzy - to increase their effectiveness in their chosen role.