For the most part, I don't see a need to single out the Wraithknight. As others have commented, most of the means of killing the Wraithknight are the same means I would use to kill any Monstrous Creature or particularly troublesome unit. I think the mobility of the Wraithknight limits your melee options to Flying Monstrous Creatures if you want to engage it in a reliable way. I suspect most people will kill it thus:

1. Shoot it with appropriate with mid to long range, high strength AP-1 or 2 weapons (or Poison) and attrition it as much as possible.

2. Engage it with deadly FMC which strikes before it to finish it off. *Or unit with poison in melee to kill it by volume.