As someone who has played against the new knight lists ~7 times now with reasonable success (5/7 wins) the best solution is to get in CC with something that hits hard, and can take a beating back
or a lot of melta fire
basically you need heavy AT in reasonable volume to do shit to them, and anything that deals targetable mortal wounds is pure gold there
most of mine have been using deathwatch, where the tempest/hellfire shells strats did work, along with twinlas mortis dreads
for Eldar, fire prisms are your best friend (or a scorpion)
hemlock is not as great, I'd personally use hemlocks for crippling armigers to keep them out of relevance then finish them off once the big stuff's dead. regular knights simply have too many wounds for d-scythes to be too effective.
Wraithguard webway striking are suprisingly good as a counter, since each hits fairly hard and knight lists will bring enough targets to make it worth it
and most lists have some kind of armor that needs to get dead, so they're not a bad TAC take either