So it seems like this is one of the latest crazy rule breaking lists to beat and I didn't see something similar discussing it. Guessing after NOVA this will probably be popping up everywhere so figuring out hard counters would probably be a good idea.
I'm an Eldar myself and really serpent spam is one of the only ideas I can come up with and even that doesn't seem all that great. Either main riptide weapon seems like it would take care of wave serpents fairly easily, especially if it's coming from a unit with an attached commander (re-roll hits, tank hunter, ignores cover). Even if they survive our normal WS config (TL-SC) would need quite a few hits to take out just one riptide. I suppose 4-5 WSs focusing on one could reliably take one out each turn but that would probably leave them all without a shield next turn and easy targets.
Everything else that I can think is either melee or has a short range, both of which are far from ideal since you'll probably get destroyed long before you make it close enough. Wraithknights with wraith cannons are probably the best choice but are expensive and you're either using tons of points or relying on roll 6s to wound.
That's all I've got so far and I'm hopefully missing something that will help. Obviously taking Tau allies would help but I'm trying to avoid that. Anyways what have others come up with for their respective armies?
Edit: oops forgot to give a sample list. Here is one very similar to the one that almost won Nova:
The List to Beat
Hq Farsight
O’vesa
3 crisis suits, bonding knifex3, missile podx3
10 kroot w hound
1 crisis suit, flamer, bonding knife
1 crisis suit, missile pod, bonding knife
1 crisis suit, missile pod, bonding knife
1 crisis suit, missile pod, bonding knife
Riptide, ion accelerator, twin linked smart missile system, early warning override
Riptide, ion accelerator, twin linked smart missile system, early warning override, positional relay
Riptide, heavy burst cannon, twin linked smart missile system, velocity tracker, target lock, 2 shielded missile drones, talisman of arthas moloch
Skyray, smart missile system, blacksun filter
Allied detachment (codex tau)
Commander, command and control node, multispectrum sensor suite, iridium armor, puretide chip, onager gauntlet, vectored retrothrusters, neuroweb system jammer
10 kroot w hound
O'vesa is the IC riptide from the farsight enclaves supplement