^ what's the rough estimate points cost for these Knight popping units?
Haywire Melee (now nerfed to hell):
Full Pathfinder squad plus grenades: 130 points (variety of other upgrades available, season to taste)
Full Swooping Hawks squad: 160
While not extremely cheap, note both of these have a number of other uses they can be put to, you're more likely to see/use two smaller squads.
Curious note: The Swooping Hawks can still (each!) make S4AP4 Haywire attacks on flying stuff they move over. They're trained to kill Flyers that way with their Haywire grenades... and it seems most of them now forget they have them OUTSIDE the Movement phase.
Melta bomb melee (also nerfed to hell)
Tankbusta Squad - see
Point of order, if you want to gib an Imperial Knight, the Sororitas still have you covered.
Repentia Squad - 155 points. Their 9 Eviscerators (Think two-handed power fist) with 2 attacks each (3 on the charge) WILL see the job done. They do not, however, have the mobility of the above units to get near the Knight - no Scout SR, no Jump packs, and no open-topped (or Assault-Ramped) transports.