Support Build
Well, it depends what do you mean by supportive – weakening the enemy, or boosting allies. As you said necro has everything, but can`t do the same support as for example guardian if he concentrates on just boons and healing. Same goes if you go with conditions, for example in you want to keep enemy in weakness state, elementalist handles this more successfully, necro has a better variety of different conditions of course, but it would be hard to constantly keep up with one of them.
Can`t say that I know successful supportive builds but when it comes for dungeons here what I use – Staff,scepter/focus, healing seed(as sylv) but basically well of blood does the same aoe heal, well of power – aoe condition removal, signet of undeath -aoe revive, plague signet* – would be great if worked properly, but corrosive poison cloud is good as well – weakness, poison, poison field, aoe, for the elite – I like lich form for its dmg, but I guess many would say that plague is better.
Hard to say something about the traits because I use condition damage build(and just switch the skills). But I`d say if your going with this skills – 10&signet cooldown/0/10&greater marks/30&life transfer heal, well cd, create a well on revival/20&DS draining slower, create a reapers mark during revival or something else.
Hope this helps at least a bit, then again, this is based on my experience in dungeons=)
Found what I was looking for. Here it is, so anybody else can have a look.
It’s a staff protection/regen build. Weakness can be gotten from other sources, or from weapon swap if absolutely necessary.