Which 3 parametrs scaling split arrow (skill gem) the most?

I want to play with split arrow, but this skill deal very small damage to bosses, so my question is what directly i must scale to start dealing damage to solo targets? Projectile damage? Crit chance/multi? Skill gem lvl?

Until you get better gear, use Barrage skill for single target.
Tornado shot is also a decent option, because the secondary projectiles shotgun.

To scale Split Arrow, probably your best bet is a lot of flat added elemental damage (bows with +Fire/Cold/Lightning damage) scaled with as many "More" multipliers as you can get (Elemental Damage with Attacks Support, elemental penetration, shock, critical strikes, curses, etc). Physical damage scaling is not great with bows.