If we think about, even free account sorcerers, druids and paladins can be somewhat relevant in a PvP situation. Mages got Sudden Death Runes, Magic Wall, Paralyze and etc, and Paladins can combo Sudden Death Rune + other powerful ammo.
But what about free account knights? They are arguably the less interesting vocation as a Free Account, since they lack even a basic exori. They rely solely on single target damage with weapons and potion healing. Having top notch skills and set may help you, but I can't imagine any decent combo or even long range damage by knights...
What resources, tactics or items can make a free account knight more useful for PvP? You can explain at what skills and level a free account knight would be somewhat relevant for a combat.