Nightblade/arcanist is pretty fun to play, agile glass cannon, my advice, have fun as much as you want the first time. Once you complete the main campaign, if you liked, buy at least Ashes of Malmoth before trying the higher difficulties, there is a potion that booster xp gain by 100% with the Malmoth Resistance faction vendor. Also, it is easy to respec, so just try everything out. Once you finish normal, respec all your points.
Just a few tips that are useful through all the game:
a) Select a damage type and focus as much as you can on it, Grim Dawn rewards especialization.
b)Find ways to reduce enemy resistances, there are enemies that are really resistant on this game and as resistances can be negative, even enemies with low resistance, lowering their resistance even further is great, you can triple your damage by just removing them, keep an eye for constellations with debuffs for enemy resistances against the attacks you use, Rowan's Crown is a great constelation for a ice nightblade for example.
c)Your resistances are essential for survival on end game, you can be one shot by anything if you have less than maximum resistances on the end game and many bosses debuff your resistances (resistance debuff appears as a broken shield), that is deadly on end game so against some bosses, you need to have extra resistances, over the cap to compensate for the debuffs.
d) Dont be stingy with all those components that enemies drop, some have really good effects that can make your life easier, use them as much as you can to make your life easier.
e)Also, if you wanna make your life easier, also, focus the first 20 or so points on a single skill instead of spreading your points, give preference for skills with AoE that are spammable and with no cooldows, each class has a low level AoE spamable skill, of course, this is more for a second character, have fun with your first one and do whatever you want, experiment at will.
f)There are enemies that can crit you for 6000 damage on higher difficulties, aim to have something like 4000 hp by the end of normal, 6000 hp on the end of elite and 10000 hp on the last difficulty.
g) Before starting a new character, make sure to grind reputation with the faction vendors, they sell armor components that are crucial to survival on end game and you can also buy writs from all of them, they will greatly speed up your next characters reputation acquisition, yes, they can be transfered.
h)Physical ability is the king of the attributes, dont underestimate the defense ability, this will protect against crits and some bosses on higher difficulties will murder you if you dont have at least 3000 of it by end game that is why on the long run, physical ability is the king, it gives you defense and life. I just dont bother spending points unless finding an item that requires it, I just hold many points for when I want to complete my build.