With the changes to Reanimation Protocals, Necron Warriors (especially in blocks of 20 models) are exceptionally durable and difficult to take down. Throw in a few Ghost Arks to regenerate additional models and a Cryptek for a 5++ save against shooting and 4+ rolls on Reanimation Protocals and many armies will struggle to deal with large Warrior Phalanxes over the course of a game.
1 – Vindicare — With the ability to choose his target (and to reduce enemy LoS! rolls for ICs to a 4+), the Vindicare can use his specialized ammo to potentially “one shot” any Crypteks attached to the phalanx, taking away both a potentiall 5++ save and reducing the formation’s Reanimation Protocals back to a 5+.
2 – Callidus — Being able to pop up right next to the formation with no scatter (or even infiltrate right next to it) and then hit it with a strong template weapon turn 1 can significantly weaken it. Additionally, since the Necrons can only snap shot at her the first turn she arrives, the Callidus has solid chance of surviving until the next turn and charging in, where she can wreak havoc in close combat.
4 – Culexus — An awesome unit, but since almost all of his abilities are best used against Psykers (and Necrons have none), he is probably least useful against a full phalanx. That said, his Etherium means it is very difficult to shoot down and he is surprisingly good in close combat, even against non-Psykers, so he could potentially tie up a phalanx in combat for many turns, thus keeping them from shooting, claiming objectives, or other important actions.