What skill is good for mobs and bosses ? I have tried almost everythign and arcanist damage is terrible without great gear early on. I have tried every skill I can unlock so far and fighting bosses is tiresome.
Any advice would be good since I have never tried this combo before, not used to casters 
Not too familiar with the Demo side of things but for Arcanist Olexra’s Flash Freeze is very nice, if you want to cheese your way through normal-vet it can carry you single handled along with a component skill for bosses. The radius and damage on it is great for the early stages of the game though it’s a pretty boring playstyle. Trozan’s Sky Shards is nice for direct damage on bosses but you need to invest in it a bit before it starts to be worth while.
I’d recommend going demo early game when leveling a sorcerer if you don’t have gear stashed. Flame strike and flashbang on top of beefier hp from demo mastery is going to make things easier.
If you’ve already gone deep into arcanist mastery, try to beeline for devastation if you can afford to respec, it’s got a ton of raw damage based on skillpoints so it will perform even crap gear. It also synergizes well with the fire damage debuff and monster freeze on OFF.
My experience with arcanist is the opposite.
It melts everything with ease, but leveling I mostly use fireblast and greater fireblast from components and olexras flash freeze.
If I want more single target damage I use aether ray or devastation or both.
Ok thanks for the help guys. I should do much better now
Fire Strike plus Flash Bang is what my Sorc has being doing since many, many, many builds ago.
I see zero reason to change this.
Flash Bang is the best damned non-attack attack in the game - bar none.
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My Canister Bomb Sorc walked through Normal, Elite, and most of Ultimate. I rushed Grenado for the first half of the game then supplemented my damage with Canister Bomb, but around mid-late Elite I noticed Grenado was doing piss-poor damage to enemies, so I specced out of Grenado and dumped everything into Stun Jacks. Quick Jacks are only an option once you have the mana to keep them going, so until you have a large mana pool, just use Stun Jacks as a supplement to Canister Bomb and nothing should ever stand in your way for very long.
Ooooo I can actually be of some help here.
I just leveled my new Pyromancer
As many other Demo’s would agree, Fire Strike line is soooo OP at lower levels its silly. I was doing 1k dmg per hit at level 18. Couple that with Flashbang and you have some nasty damage right there for the earlier leveling portion.
I cruised through the early part of the game with astonishing ease. Easier than any other build that I’ve played so far.
So yea, that would definitely be my advice, use the Demolitionist early on and if you really want to change to Arcanist main you can make that switch afterward. I would at least level up the FS line until 25.
This…by a mile.
If focusing on Demo…
Grab 2 pistols (the ones from the Legion shop will do) and Gunslinger’s jacket (or talisman if you don’t have the jacket) and use Brimstone (last skill in Fire strike tree) and static strike if you have extra points. Flashbang is nice with only a few points invested, but get Flame touched maxed asap because the OA bonus is amazing. Don’t bother with Fire/Explosive strike, they just increase the mana cost for very little early game benefit.
If focusing on Arcanist…
Proliferation (last skill in PRM tree) and a few points in Distortion for the piercing, don’t invest too many points or the mana cost will kill you. There are a ton of early game items with aether and Arcanist bonuses, and the Crossing/Legion shops have some stuff as well. Proliferation, Mental Alacrity and Reckless power will carry you to level 60 easily with self-found items. Level 1 Devastation works wonders on bosses.