I've tried making this work and leveled my first Commando with this intention but there are several problems. First of you'll need to invest a lot into Demo, i'd wish to stay in 32 but you need go full 50 since skipping Brimstone isnt an option and dps already needs every boost it can get.

FS line is just too hungry, needs so many points and investment. Which leaves us with the problem of investing in Soldier Tree. You'd like to use an Exclusive skill, Menhir's Bulwark is out of question so Oleron's Rage, its cool to have 12% MS and 12% OA but going full on Soldier Tree and more points into it seem to be a waste plus no utilization of Pierce and IT damage.

After semi sorting this out the gear becomes another thing to consider. Weapon choice would be hard but maybe some Rares with good affixes would solve it. Ulzuin is intended for ranged since 2H Rifle weapon is part of the set. Maybe 4 Piece Ulzuins or 3 piece Ulzuin + 2 piece Justicar. You could get +1 Demo from Justicar Helm, Belt and Relic.

Overall tankiness and dps is a big problem despite investing a lot of points into FS. Cadence based Commando would have a lot more tankiness, dps output and points to spread, you could try sword and board or just 2H with link and Justicar set, pretty high dps and armor + armor absorption and DA/OA.