Lascannons are marginally better than Missile Launchers for MC-killing, I find--the main MCs I'm shooting at these days being Riptides and Wraithknights. Lascannons have better strength than Plasma for killing Wraithknights, but better AP for killing Ripdtides, and are only a bit more expensive than missile launchers. Krak missiles are nice and all, but Lascannons will melt basically anything (and wound Wraithknights more often than missiles in the grand scheme of things).
An allied Plaguebearer squad can probably put some serious wounds on with its poisoned attacks, provided it could ever reach the Wraithknight--being pretty damn slow, that strikes me as slightly unlikely. Plague Drones then, perhaps? But that's digging into allied Daemons. Mass Daemonette/Seeker rends also work.
If you've put a few wounds on a Wraithknight, a Black Mace Daemon Prince should be able to charge in and finish it off--but if it doesn't kill the 'Knight, the 'Knight will strike back and Instant Death it at S10... and if it does kill it, your Prince will be left out in the cold, vulnerable to enemy shooting. ...get the enemy to charge YOU with the Wraithknight, perhaps? Somehow?
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!