Why do you keep posting in this thread, when you obviously don't PvP that much?
You realize that every other class can get guarded too, right? And be more productive?
LOL even @1 knockback. When I face a class with one I usually wait for my fellow knight to leap towards the target he punches knockback and I leap immediately after that and my fellow knight has time to reach the target and get few shots too. I bet a merc/commando comes right after that on forums to say a few words about warriors/knights
EDIT: I have no idea how to improve the AC without redesigning their dps trees, but escape ability like jetpack for mercenary and... rocket jump(ha! quake) for commando would sure make pyro/assault and heal spec a lot better, mobile and cooler. Arsenal/Gunnery would need a complete redesign or making tracer/grav instant.