My character is a Soldier using melee weapons. So far i haven't decided to assign another class to him. Do you think it is viable? I just want to develop Blade Arc to be as powerful as possible before moving on

It can be done, but you need really good gear and you sort of gimp yourself because you're not getting the benefits of stats from a second mastery bar at the minimum.

Here's an old Soldier only build

https://forums.crateentertainment.com/t/1-0-0-6-pure-soldier-op-cadence-and-break-morale-in-action/35786

and a more recent one

https://forums.crateentertainment.com/t/1-0-7-1-single-mastery-soldier-build-no-greens-used-170-clear-4-buffs-no-banners/48512