Soldier is great support class for a Demo build. It gives:
- HP! 1 point in Soldier Mastery gives 6 Physique, vs 2 Physique for Arcanist Mastery
- toughness (Military Conditioning, Decorated Soldier, Scars of Battle)
- boost to damage (Squad Tactics), and Offensive Ability & Movement speed (Oleron’s Rage and to some extent Field Command)
If it’s your first character then Soldier is a safe choice, especially when the game gets a bit tougher in Ultimate. It’s also easier, in my opinion, to make a good end game character (Nemesis boss hunting / Crucible) with Commando rather than Sorcerer.
For devotions, for levelling with a 2H weapon, you want to go for Kraken ASAP as it gives a massive boost to 2H damage. The quick route would be:
+1 Green
Scholar’s Light
+1 Blue
Eel (3 points) or Sailor’s Guide (4 points, but more useful early on IMO)
Kraken
(Check if you need to keep Hawk for the reduced cunning requirement for ranged weapons)
I would also take Viper fairly soon for the resist reduction, and then head towards either Rhowan’s Crown for the Elemental Storm de-buff (Empty Crown is a good choice to get the affinity), or Soliel’s Witchblade for Eldritch Fire. Bind one of those to Fire Strike.
There are lots of final devotion setups you could go for; if you go all offensive, which works for levelling, then start building up points to get Ulzuin’s torch.
I have a fairly defensive setup on my 2H ranged Commando, but I wouldn’t necessarily recommend this for levelling unless you are struggling: https://grimcalc.com/build/1006-lAWU7H
Flashbang is great…make sure you take Searing Light as well to reduce enemy damage. I always like to max out Flashbang if I have the points, but don’t take Searing Light past 7 as it doesn’t scale well after that.
Thermite Mines are good on bosses for the resist reduction on Hellfire Mine, but skill points can be in short supply on a Fire Strike Commando, so I would only put a few points in and try and boost it from gear. They are also just annoying to use on anything but bosses 
Your main damage should come from Fire Strike and Brimstone, so boosting things like Canister and Grenado is a waste of time really. Once you have Fire Strike line maxed I would aim to get Oleron’s rage to 12/12 myself.
These are just my own thoughts, but hope that helps.