Arcanist. Mental alacrity, Inner focus, Elemental balance and of course the Sphere that keeps getting nerfed are all amazing for supplementing a spamming caster build.
People saying Nightblade are horribly wrong and making a poison Witch hunter when you don't already have godly items to give him will cause a lot of frustration and a restart!!!!

DEE (dreeg's evil eye)[grimdawn.wikia.com] - Duh... Your main attack. Don't max everything right away because the mana cost will be insane, max Vile eruption as soon as you can.
Possession [grimdawn.wikia.com]- Improves all of your main dmg types, gives you protection and flat chaos dmg which will be high for you anyway. Max very early the game when +125% dmg is huge.
Blood of dreeg[grimdawn.wikia.com] - Ton of offensive ability and health regen, spammable heal, extra acid base damage, holy ♥♥♥♥ this is insane. Max very early to fix your offensive ability. Amor of the Guardian is a good place to put extra points after lvl 80.
Bloody pox[grimdawn.wikia.com] - The best devotion triggering skill in the game, attach some low cool down proc and watch it go off like crazy after one cast. Bad skill on it's own.
Curse of fragility[grimdawn.wikia.com] - Base skill is useless, though you may want to put a few extra levels early for larger are and slow, respec later when you have +skills. Vulnerability is your main resistance debuff for certain enemies with high psn/vita resistances (leeches, aethereal bosses).
Solael's Witchfire[grimdawn.wikia.com] - Main skill useless, but second rite is where you can put points if you have spare.

Inner focus[grimdawn.wikia.com] - Massive bonuses to spirit (+ all dmg for you, lets you equip amulets) and offensive ability. available at lvl 1. Insane. But go ahead, pick demolitionist for a hard to target spell that gives almost as good resistance reduction as Manticore after 40 points invested in the mastery... I'm sure it's just as good. I was sarcastic, it's not. Invest points as spirit requirements for amulets get higher or as it becomes harder to crit things. Don't put actual points into spirit!
Mental alacrity[grimdawn.wikia.com] - Cast speed bonus got nerfed, though it's still good, obviously. The main reason you want this is the skill cost reduction, allows you to max the entire DEE line without running out of mana after 5 casts. Max before maxing DEE synergies. Stacks with -cost bonuses from items (hint!).
Elemental balance[grimdawn.wikia.com] - Crit bonus. It got nerfed everywhere except on this skill. Poison now cries as well, making this skill even better for this build than it was in previous versions.
Maiven's sphere[grimdawn.wikia.com] - Regardless of the constant nerfs, it's one of the best skills in the game. Stacks with Possession.
Arcane will[grimdawn.wikia.com] - Damage bonus when you need it the most, still not worth investing more points imo. Get the one point as soon as you can.
Nullification[grimdawn.wikia.com] - Destroys Cronley and saves you from killing yourself on Aethereal mages and reflect bosses. You will rarely use it, but if you don't have it you will regret it.
Olexra's flash freeze[grimdawn.wikia.com] - Great when surrounded by mobs, like in the Loghorrean fight.
Calidor's tempest[grimdawn.wikia.com] - Same as Olexra's, but works on different things. If you can knock down a mob, you can usually knock down a boss of the same type (skeletons and Cronley's gang for example). Needs the transmuter to be useful, so don't get it before you have 10 points in mastery.

I still haven't done all the testing, but the devotions you definitely want are:
- Hawk[grimdawn.wikia.com],
- Viper[grimdawn.wikia.com],
- Scales of Ulcama[grimdawn.wikia.com] (it got nerfed but I think it's still good for builds with high vitality dmg)
- Behemoth [grimdawn.wikia.com]and/or Targo [grimdawn.wikia.com](4 points for health).

Other good ones are:
- Chariot[grimdawn.wikia.com],
- Obelisk of Menhir[grimdawn.wikia.com] (3 points for +300 armor and +5% defensive ability; don't forget to put Scaled hide into your pants and shoulders for absorption),
- Eye of the Guardian[grimdawn.wikia.com] (take 1 level into Bloody pox and it's synergies and attach this devotion to it, the eyes will spin like crazy and kill anything that comes close)
- Tree of life[grimdawn.wikia.com] (insane health bonuses and the heal is really good, especially with a lot of cooldown reduction you will have)
- Oklaine's Lantern[grimdawn.wikia.com] (only take it if you already have the requirement, it's not worth investing into green devotion just to get this)
- Huntress [grimdawn.wikia.com](4 points for 3% offensive ability, only if you really lack oa)
- Manticore [grimdawn.wikia.com](not as good as it used to be, put it on your pet or Sigil if you use them)
- Wendigo [grimdawn.wikia.com](I used this on DEE with my warlock and I can destroy ultimate without any lifesteal in B30, i have no idea if it's still as good)

Do not get:
- Dying god
- Abomination
They are not worth the huge investment required

Well, so far I find 2 main advice on this:
1. All physique with something in spirit and cunning for gear - for example like this:
Physique 60
Cunning 8
Spirit 13
2. Physique and spirit 1:1 or 2:1 and no cunning

Kymon[grimdawn.wikia.com]

This build destroys Order nemesis because the eyes confuse him letting you just attack him for free. If this is your first char that gets to ultimate, this nemesis can be your source of easy legendaries. He also sometimes spawns with Legendary daggers and off-hands which are good for your build.

Example build[grimcalc.com]
Most of the content I take from here:
grim dawn forums[http//grimcalchttp]
This build, except attributes was suggested by GeneralCash, so most of the credits go to him.

P.S. I want to add pictures soon, but dunno how soon I will have time to this again.