Well, if by anti-Nid you mean shooty, then the following is best (if pricy);
Deathwatch Codicier, bolter, force weapon, psychic hood, FotA
2 x Deathwatch Specialists with heavy bolters, suspensors and Hellfire rounds
2 x Deathwatch Marines with bolters and Kraken rounds
With the exception of the Librarian's 'magic line of repeated S5 death', every weapon is AP4. Hellfire rounds are the single nastiest weapon you can employ vs Nid's; nothing except the Broodlord, Tyrant, Guard, Carnifex and Zoanthrope can have a save against it. Every other Nid unit will be wounded on a 2+ and have no save (Lictors being the one annoying exception, as they have a 5+ cover save in the open). Yeah, if its wasting Nids, thats the ideal shooty squad.