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.

Gear can carry a bad/mid skill but really you should probably just have a 2nd skill that's more made for single target dps and just use split arrow for aoe clear.

Split arrow is entirely scaled on how your build performs.

Are you elemental? Then it would scale with flat and % elemental mods, along with pierce.

Are you physical? Then it would scale with physical

Are you bleed? Then it would scale with DoT

etc.

The problem with split arrow and single target is that it has low damage effectiveness and no built in damage. So it will never perform well in single target no matter how you scale it.

By comparison:
1. barrage pn a single target deals damage 7 times @ 54/55% for a total of nearly 400% damage effectiveness
2. Split arrow on a single target deals 142% damage effectiveness.

No matter how you scale it, barrage deals 2-3x more damage without changing anything. Add to that better single target supports that are used with barrage.