Going Necro-only will be tough early game, as you won't have access to many damage skills. If you want to make a beast from your Necro, go Warfare+Necro, it's devastating, but only after you can cast Grasp of The Dead and/or Blood Storm.
Necro/Summoning will work great (go for 10 summoning first, then go Necro). So in your case, you can just pump into summoning and make a blood summon.
I found Scoundrel to be very fun and powerfull, as you should invest into scoundrel and warfare, you have access to a lot of CCS and they will outdamage your other physical character at first (but will fall off in the mid-endgame)
I'm at the ship so I can respec now my two elemental mages to something physical

So I'm keeping my warrior/poly and my pure ranger.
I've added a Necro/Warfare before seeing your advise so it makes me think that is a good idea!
For the 4th, I'll be doing a Necro/Summoner but I have two choices here:
- Relying on the Bone Widow (little preference here)
- Relying on the Incarnate
Those are 2 very different builds, even though they are both Necro/Summoner, the spells and side schools choices are very different.
Still wondering ahah.