Having killed a few knights, what I find works:
Rest of army
Your Riptide presents the forward element and at distance engages the the Knight. Your army hides and evades the knights fire and keeps about its day winning the war.
Your deathrain team is positioned to flank or Deepstrike.
Melee is where this monster gets its points back. NEVER let that happen.
He is also often played as "the spear" of a force, so side flanking is fairly easy.
Base size alone makes him check for across field movement for most of his moves, you have 6 Plus JSJ for your forces.
I have also had some effect with 2 sky rays/ 2 hammerheads. Rays on flank (like Deathrains) and the Hammers split up but on point. The invuln sucks, but rail rounds can score some sick damage fast. And at mid field, 2 full seeker batteries finish the job if the hammers did not. Dpods and cover render most of its fire useless.