As a tank, Knight relies on his debuffs more than any defensive ability. Power Break cuts a target's damage in half, and reduces attack speed. Speed Break reduces nearby enemies' attack speeds to the minimum. The cooldown of these abilities are equal to the durations, so you can always keep them on the enemies.
If fighting multiple enemies, Power Break the stronger one and Speed Break all. If the enemies are the same type, Power Break the one that your team is not focusing.
Equipped with tank items, Phantom Fighters can also be potent tanks. If tricked into attacking the phantoms, then you essentially have three extra Knights to tank, and there are no consequences if they die. They only take 25% extra, which is a fairly small amount for many illusions. The main vulnerability for this strategy is AoE abilities, which will crush the phantoms.
Knight has a potent damage debuff - Armor Break, which reduces 75 armor. This is enough to nullify any Protect spells put on field enemies, or take a good amount of armor from a boss.
Note that Tonberry and Armor Break share the same buff. If you use Armor Break on a unit that was hit by Tonberry, it will overwrite the Tonberry armor reduction with the much lower Armor Break. However, you can take advantage of this glitch by first Armor Breaking the enemy then using Tonberry. That way, you get the higher armor reduce of Tonberry and the higher duration of Armor Break.
Phantom Fighters will effectively increase the Knight's damage output by 225% (though in practice it can be lower, due to how images are handled by WC3). Pair this with Tonberry to easily cut down any enemy.