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I know that my favorite opponent is in the process of gathering enough terrain for some good sized Cities of Death games and I'm also eager to give the Necrons a try in built up territory. However, I'm not quite sure if my normal list of warriors, destroyers and monoliths is working in Cities so I'd gladly like to hear your experiences and comments on what are the best units and tactics for Necrons to use in Cities of Death games.In case it matters the most likely opponents I would be facing are Space Marines (standard rhinos, tacsquads and land raiders setup), IG (horde), Tyrannid (horde+CC) and Eldar (guardian+mech).
Not so much for the 'competitive side' of the game but for pure awesomeness... WRAITHS!!!
They are so much fun to use and are extremely valuable thanks to their large movement range and ability to ignore impassable terrain. A unit or two of them will force your opponents to seriously reconsider where they put their heavy shooters.
Other than that, it's pretty much the norm although some units work better but warriors are at more of a risk of getting cut down as 10 man squads can get a bit awkward in narrow streets.
Necrons W68-D13-L10 (3rd edition codex)
Necrons W3-D1-L1 (latest codex)
Skaven still being assembled
Having only watched CoD games, I've seen Wraiths wipe out some pretty large squads of anything they've run into. Another idea may be some Scarab Swarms for the sake of granting Warriors cover saves (if you plan on playing the Warriors defensively.)
I've heard (never played CoD actually) that pariah's are awesome in Cityfight-ish battles. Many coversaves, good shooting and decent combat. Maybe you'd throw in some of those.