I have been scratching my head for a while trying to come out with a viable DE counter to Wraithknights, given that they are now basically immune to poison weapons with their Gargantuan Creature status, 3+ save, and multiple wounds.
Outside of massed Dark Lances (which means using almost your whole army to focus on a single Wraithknight… not a good formula for victory), the only real solution I have come of with is massed Talos Pain Engines. More specifically, the Corpsethief Claw formation, which consists of 5 Talos engines, all in a single unit with the Scout USR and Freakish Spectacle (-1 Ld to nearby units) rule.
Scout helps the unit get accross the board to get close to the Wraithknight and the 15 T7 wounds with a 3+ save and FNP should enable them get accross the board without being shot to pieces. Between Scout, normal movement, and running, the Corpsethief Claw can get betweem 13-18″ accross the board by the end of turn 1, setting up turn 2 for a 6″ move and 2D6″ charge for a decent chance of “catching’ the enemy Wraithknight in close combat.
Now, the key is to upgrade all the Talos with the Inchor Injector CCW… this gives each Talos 5 x S7 AP2 (they MCs, remember) Fleshbane attacks on the charge (so 25 such attacks if you keep all 5 Talos alive), all at WS5 and with ID on 6+ to wound. Translated to combat against a Wraithknight, say 3 Talos survive long enough to attack, that’s 15 attacks, about 10 hits, 8-9 AP2 wounds (of which, an average of 1-2 will have ID), and 5-6 unsaved wounds after Wraithknight FNP rolls — throw in the extra wounds inflicted from the ID results and you should have a very good chance of killing the Wraithknight in a single round (or two at the most) of close combat.
They key, of course, if catching the agle Wraithknight. Cunning positioning of your units and using the Corpsethief Claw as you “anvil” can trap the Wraithknight and allow you to charge it. Even if the Talos don’t successfully catch him, they can absorb LOTs of Eldar shooting (including from the Wraithknight itself) and threaten the rest of your opponent’s army, thus providing good utility even if they don’t accomplish their primary mission as “Gargant Creature hunters.”