- First of all, their sweet spot is definitely 24" or closer. That is when most of their weaponry is most effective, so staying out of range of that is important.
- While their Reanimation Protocols are improved, they cannot stand up to focused firepower. Many of the Necron's armor saves were nerfed, so "killing" them is easier to do. Shoot them and shoot them some more. :)
- To help ease that problem, Necrons feature several units with the ability to grant Nightfighting to the battle. Solar Pulse wargear and Imotekh the Stormlord are main methods. So if you're able to get a couple of Searchlights off, then this advantage will cease to exist. However, its important to weigh the pros/cons of using a Searchlight to the situation at hand. Sometimes using Nightfighting to your own advantage is beneficial.
- They still suffer from initiative 2 on the majority of their units. If you can manage to get into assault with them, even with a 5-man squad, you'll likely take them down in a Sweeping Advance. However, the downfall with that is you're likely going take heavy incoming fire the next turn. To couple with this, very few of their units are worth their salt in close combat, so you'll have little difficulty winning combats.
- All of the Necron's vehicles are relatively fragile. Sure they have Quantum Shielding, but if you can simply put a single Penetrating Hit on them, the shielding goes down permanently. Then AV11 (usually open-topped) is what you're working against. Taking high strength weaponry like Lascannons and Tank-hunter CMLs will work wonders here.
- The Necron's are also extremely slow. They have some transport capability, but taking significant numbers aren't really cost effective. So they will likely be footslogging around the board, making them vulnerable against highly mechanized forces who can redeploy rapidly. Which leads me into my next point...
- Necrons seem to have a slight problem busting open armor. At least in the shooting phase. The only unit that really has that ability is either Crypteks with the Eldritch Lance (36" Str 8 AP2) or the Triarch Stalker. Necrons can open armor in the close combat phase, however, with Canoptek Scarabs and with Warscythes (especially Command Barge sweeping attacks). Beyond that, there isn't much that can hurt you there. So if you're able to neutralize those units, your mechanized forces will be able to influence the battle dramatically.
Stay mobile to stay out of range and focus your fire on the most dangerous pieces of your opponent's list. Usually this will be Command Barges, any Heavy Support vehicles, and small sized CC units. Just keep firing and jump into close combat when the time is right to bust some robots up. Use bubblewrap if you have to, but make sure to stall the giant line of Scarabs for as long as possible.
Anything else I should add to that? Something I'm missing? Disagree with what I said? Let's hear it. :D