For Kill teams, Necrons are actually a little easier to kill than if you were playing a regular force. However, in a straight firefight, you'll almost always lose due to his Super Feel No Pain/ Reanimation protocols.
However, your Marines have more gear and weaponry that you can take that he simply doesn't have options for (as long as he is just deploying warriors anyways).
I recommend for the first game maybe even two, you just take standard boltguns, to see how pretty bad they are (at least towards warriors). Unless you are taking chapter tactics Imperial Fists to get those sweet sweet rerolls. Then boltguns are actually pretty decent.
Once you guys play a few games and want to be more competitive. Necron Warriors are super vulnerable to Heavy bolters. Wound on 3 and ignore their armor causing them to go straight to RP rolls. Plasma, grav and melta are all good, but point for point against warriors are inferior. Note I say against warriors here. As Plasma is overkill and can kill you, Melta is short ranged but great, grav will wound them on a 4+ making it in this particular fight worse than all the prior weapons (still better than a bolt gun though)
If going all ranged is not your thing, you'll overall do better in melee when fighting the necron warriors, as you'll always strike first due to their poor initiative. A power maul and grav pistol will make your sergeant a mean melee fighter who will ignore all of his warriors saves in the shooting and melee phase.
A storm bolter is not a bad option either as its cheap and gives you more range and an extra shot.
Don't forget all your marines have krak grenades. If you are within 8" of a warrior, instead of shooting their boltgun or pistol have them throw a krak grenade S6 AP4. Wounds on a 2+ ignores their armor save, plus you can still assault if it survives.
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