Tesla spam in 5th was about putting as much tesla on the table to simply destroy vehicles through weight of fire. The issue was Tesla weapons maxed out at S7 and had AP - and unlike Grey Knight Passback based armies, that S7 didn't have Rending. This meant dealing with higher AVs consistently could be difficult for Tesla-based Necron armies and thus Command Barges, Scarabs & Crypteks to shore this up. So how does this translate to 6th edition...
The core of the army got way better in terms of ability to inflict damage and the ability to take return damage on the chin. Does it equate to an equally familiar list though?
- Hull Points is huge. One of the biggest criticisms leveled at any Necron list was being able to drop vehicles in a guranteed manner. There is no such guarantee obviously but being able to throw a handful of meltas at a vehicle was way better than trying to strip it down to scrap metal. With Hull Points, that stripping down concept works. Vehicles still die to the relative same number of penetrating hits before but now glancing hits are reducing damage, the variability of dice is taken down a notch and weapons which weren't as powerful before, i.e. AP - weapons, are just as good now.
- Flyers is also huge. It increases the durability and speed of both of the Scythe variants for Necrons and actually makes the Doom Scythe relevant. Lack of Hover or a turret mounted weapon means there isn't 100% applicability here but these guys are very much a force to be recokned with.
- Night Fight has changed from a "be hit or not hit at all" system to a cover based system or not hit all system (36"+) and is a lot more common with the 6th edition missions. Necrons still have the ability to turn the lights on or off and combined with the extra Night Fight in games, will have more flexibility than their opponents here. They still suffer from Night Fight in terms of not being able to combat it directly but they can take signficiant advantage of it and have more control when it can be used.
- Two of Necron's main weapon types, Tesla & Gauss, got nice boosts. Again for Tesla, the lack of AP - negatives on the vehicle damage chart is great but also the increased importance of 6's to hit - i.e. snap-firing and overwatch. This means in those cases they do more damage than many other weapons. Gauss with Hull Points is obviously more effective as well - capable of glancing vehicles to death again though it requires a lot of shooting, it can help take off the last hull point or two of a vehicle.
- Which with the new Rapid Fire rules, is why Warriors are better overall. They can be effective at greater than 18" finally.
- Random Charge Length does hurt Necrons though. They suck in combat and RCL brings in the ability for an opponent to get larger than normal charges off against you. Yes reliability wise most units are looking to get within 4" to make sure they make it but you'll commonly see 7-8" assaults and for an army which loathes close combat, those couple extra inches, even if you get the extra firepower back at them, can be the difference between a dead and a live squad.
- Scarabs though make quite nice bubble-wraps now due to Fearless. They're not going to disappear under a hail of No Retreat wounds though their resilience to shooting has gone down without buffing their save somehow.
- The Royal Court is no longer a necessary evil times two. One is certainly a good idea because of the utility options Crypteks bring but you no longer need to take two just to ensure you have a solid ability to take out vehicles. This saves a LOT of points which you can put elsewhere.
- AV13, even psuedo-AV13, is pretty awesome in 6th edition at range. It prevents a lot of Hull Point damage and if any of that shielding can last until the end of the game, they're going to be a tough nut to crack. They lack the massive range of some of the other armies though (outside of the Doomsday Ark) so that AV13 is more melta/combat vulnerable and once it's gone, it's gone.
- Necrons have access to some of the best Allies in Tau, Grey Knights and Orks. Use them wisely.
So what sort of template are we looking at here?
Necron Overlord w/Warscythe, CCB
Royal Court; 2x Harbringers of the Storm, 1x Harbringer of Destruction, Lance, Solar Pulse
2x5x Warriors w/Night Scythe
2x Annihlation Barge
Totals: 970 points
Looks more like an Allies template doesn't it? The basis of the Ally template is certainly in there - double small Troops in Scythes + extra flyer and Command Barge but there's a lot of room to expand here even before you get to allies. We've already maxed out the Heavy Support slot and probably have as many flyers as we want - though we could get one or two more if we really wanted. We've already got some utility in the Crypteks but have more capacity there but there's really little we can do with HQ other than upgrade the Lord to Anrakyr for Mind in the Machine again.
Otherwise how else is this to be expanded - particularly in terms of Tesla spam? Here's where the metatransition falls apart a bit as the best way to get more Tesla is through Night Scythes but too many of them becomes a list which has the issues associated with mass flyers. You may be able to laugh at a lot of armies but some really dumb armies can just laugh back at you - not to mention what happens when you have lots of points shoved into vehicles and not enough into ground units. You could counter this with maximum scoring Allies (2x10x Strike Knights for example) but it's unlikely to be enough.
Regardless, beyond potentially more Night Scythes we really need to expand our Troops and check out the Fast Attack slot for Necrons. I'd be looking at Warriors + Ghost Arks or larger Immortal squads, potentially with a Veil, on foot. You'd need combat defenses here though like Scarab Swarms or through the use of Allies in large Ork Boyz squads. You could also change the original Warriors to be Immortals and if you're looking to add one or two more Night Scythes, I'd particularly consider this role so their contents are a bit scarier. A large Foot Warrior squad supported by smaller ones in Arks + some Royal Court members also isn't going to be a bad idea as discussed by Biscuit for 5th. Still needs combat support though.
Otherwise Scarabs are still a halfway decent option of scaring Heavier armor though certainly not as necessary as some thought before. They also act as a buffer between the rest of the army and any assault elements the opponent may have. Wraiths can also work in this regard but are more expensive, less effective versus vehicles and still cry foul at the sight of Missile Launchers and the like. They annoy the shit out of some units but aren't superwtfomgamazing. Tomb Blades bring mobile firepower to the board of different type. Destroyers still aren't great - yay for re-rolls but why do I need to kill Marines when I already kill infantry? Particularly at that price. And with hull points coming in and Tesla being very good in this regard, Heavy Destroyers become less necessary again. You could take a few just to even out your firepower base but you could probably get more and cheaper options like this through your Allies (hey, do Tau have like Railguns or something?).
There's really a ton of options here. A ton. Because you're using barely half your points in a 1750-2000 point game to fill out the core of your army and that's if you're wanting to stick with the Tesla-spam concept. You could drop the Barges and go for Doomsday Arks and/or not bother with the Command Barge, etc. The core of the list is essentially what is now the Ally template with some of the better Necron choices added on (Royal Court, Barges, etc.). What you do from there is really flexible and can almost end up looking like any other army list.
The key thing to remember is: don't over-do it on the flyers and if you start taking more than three, ensure you have a good ground force to fight over board space.