First off - don't take vehicles. That's just asking for trouble.
Beating Necrons is as simple as overrunning them in assault and using sweeping advance so they don't get their
RP rolls. Blood Angels have the perfect tools for this - Death Company put out a stupid number of attacks, and Sanguinary Guard will knock down just about anything in the Codex without allowing it an armour save. These will give you enough of a victory that the Necron player is unlikely to pass their Leadership test.
You need to make sure you get the charge though, and that you stack the odds in your favour to win and sweep them on that charge, so jump packs are the order of the day. Chaplains also do well here, particularly since the maul will still ignore standard Necron armour.
One thing you must watch out for is Mindshackle Scarabs. Where possible, attack with a unit with both a Sergeant and a powerful
IC attached. You want the Sergeant to challenge (or accept), get hit by the Scarabs and do only a couple of attacks against yourself while leaving the
IC free to smash the unit, so you still keep the advantage.
Lastly, your bog-standard Assault squads with meltaguns will make a mockery of any vehicles the Necron player decides to bring along. Take a few meltas - perhaps even Sternguard in a pod? - and Deep Strike them in.
To be more specific on the units you mentioned - C'tan shards are usually easy enough to avoid but if he takes Writhing Worldscape, it's a right pain in the backside I'm afraid. Monoliths are easily melta'ed away. Deathmarks aren't actually that much of a pain against Marines unless taken in large numbers since you still get your armour saves and any
FNP floating around from
DC or Priests, just assault them like any other normal unit. The Overlord with veil could be a pain but if he's with a unit, they'll have a big enough footprint that you *will* be able to catch him up and assault.
The only thing you'll really have trouble with is Night Scythe spam. Our only real defences against that are Stormravens and fortifications, but really you'd have to know you're facing it in advance to want to take them all.