I would pick any two-handed weapon preset at character creation. That way during you'll get a two-handed weapon on the ship during the beginning.
Edit the preset to have one point in necro and one in poly.
Take Blood Sucker, Decaying Touch and Bull Horns.
Take Opportunist as your starting talent. Make sure take take Leech as your next available Talent.
Focus stats are going to be STR/CON. Put att points into Memory/Wits only when needed.
Put one point into Hydro early so you can get the spell Raining Blood as soon as it's available.
You only *need* two points into Polymorph to get the spells you'll *need* which are, Spread Your Wings for more mobility and Heart Of Steel for more survivability. Feel free to pick up Tentacle Lash and Chicken Claw for cc/utility if and when you have the extra cash laying around. It's not a bad idea to get at least three points in Poly as Skin Graft and Summon Oily Blob are good spells too.
All other combat points should go to Two-Handed, Perseverance and lastly Poly (for more att points).
To me this is the most synergistic way to build Necro/Poly on one character (Without considering other party members of course). The primary goal will be to wear down enemies physically and dish out damage with two-handed weapons. Between Raining Blood and Opportunist there should be enough blood terrain for you to sustain with Blood Sucker/Leech. Poly will help you move around the battlefield with Bull Horns and Spread Your Wings, CC with Tentacle Lash/Chicken Claw and sustain your phys armor with Steel Your Heart.
If you want a build that is closer to a hybrid spell caster I would either go Scoundrel/Necro/Poly or Hunstman/Necro/Poly and focus FIN/INT. If you already have someone (or are going to) that deals a lot of physical damage I would go dual-wield wand and focus on spells that have statuses.
I'd been more than happy to go into more detail on the other three builds. I just didn't want to make this one post too massive so I focus on one. >3<
