While Necro has a challenging rotation, it is one of the best DD classes this patch.
Class Mastery
Nothing Wasted procs on consuming a corpse and gives you up to 20% extra weapon and spell dmg at full stacks. Also increases max health.
Cycle Unending procs when one of your Necro pets expire which should be all the time, and grants you multiplicative dmg done up to 25% depending on your targets health. For instance, if your target is at 50% health, you'll get 12.5% extra dmg done (or so I think).
Skills
Prebuff is Archer, Blastbones into Stampede then bash for Warmask.
Blastbones must be cast every third skill or thereabouts to get the most out of it. Corpseburster is also where you will get the most of your damage as it turns Detonating Siphon into a spammable as long as a corpse is up. If there is a corpse, SIPHON.
Venom Skull should only be cast when it has its third cast proc as this increases its damage and creates a corpse. During a rotation, I would recommend casting blastbones-siphon/boneyard-skull-siphon and repeat on front bar.
With large bosses such as the Dragons in SS or Taleria in DSR, Ghostly Embrace can replace skull as hitting with each GE area brings more damage than a third cast skull and also generates a corpse.
Carve must be refreshed before it runs out otherwise you'll have to get your 3 stacks up again. Barbed trap can be allowed to expire but not for long.
The direct damage increase from the Maelstrom greatsword lasts 18 secs so the 15 sec stampede doesn't have to be refreshed immediately if it falls off.
Using Banner greatly simplifies the rotation but could be replaced with Travelling Knife (multi target, warrior's opportunity and off balance) for more martial damage and Summoner's Armour for extra sustain and corpse gen.
At least one Necro DD will need to run Glacial Colossus for the major vulnerability in bosses which will be extended by Nazaray from the main tank. Other ults that could be used include Pummelling Goliath which gives the highest damage a Necro can do but requires bash weaving which is very difficult and not recommended on console. Meteor usually brings more damage than colossus but less than Goliath.
Sets
Necro is carried hard by Corpseburster but luckily it can be front barred as all the corpse consuming skills are there. This set is compulsory for a Necro DD.
Null Arca is good for single target fights. I have found that Coral Riptide works just as well but I am a serial Carve/Trap overcaster so if your rotation and sustain is really good on Necro, do NOT use Coral.
Sul Xan replaces Null on add pulls, just make sure to pick up the little soul that spawns. Necro is lacking in crit compared to some classes so this set makes up for that in add pulls.
During add pulls, huntsman's warmask is replaced with Corpseburster hat and Shattered Paths Signet since the colossus can be dropped as major vuln is sourced from elsewhere.
For the alternative setup, I've put in a second Spaulder alongside the barrier healer as Necro should have very good sustain.