I'm not completely up to speed with all the rules, but would the bonus from Rending help Slaanesh Daemon princes penetrate the knight. Maybe even a full squad of slaanesh heralds on steeds with greater etherblades?
The Heldrake won't do much to take down a Knight. It requires 6's to pen it if using Meteoric Descent.I'd suggest flying units. Preferably allied Heldrakes, as it is weak against air.
Unfortunately, this doesn't mean they won't have a dedicated AA unit unless the person is opting to be a bigger dick by having 3 Knights.