I believe it's safe to say that the Scatter Shots are among the most overpowered skills out there, and a big contribution to making bowies the unbalanced class they are.
And while a hunter can amass a lot of physical and pierce damage on the arrow itself, having each hit/puncture scatter about 10 fragments of 200 pure piercing damage -increased many times- around is what really devastates enemy groups.
(If you want to see by how much, try it on a speedy low-damage bow.)
Now the balancing could be applied to the scatters, the pierce damage boni on the auras, or both.
But anonther idea just crossed my mind: Change the damage type to bleeding.
First of all, even if the game is not logical, it does make much more sense:
While a focussed thrust like an arrow or a spear is meant to pierce armor, flying bits and splinters are what it should be best at protecting against. Unprotected areas, otoh, would be ripped open and bleed.
Secondly, it caps the damage, because it is harder to reach astronomical DPS just by concentrating on piercing. Instead the hunter would be required to develpo his bleeding skills to be good against groups, which is to be encouraged anyhow.
And third and most importantly, DPS does not stack with itself.
While it may be objected that someone already bleeding is in fact not protected against bleeding even more, this does prevent the effect that larger groups make a single arrow so much stronger. - Some Satyr being hit by the shrapnel from the guy in front, his own, and the one behind him is what causes the mass deaths, because one arrow "explodes" multiple times without losing power.
So if every monster hit by one or more fragments would bleed for a few seconds (for the same damage, which is to be adjusted), this would retain the hunter's capability to fight groups but prevent the "piercing volley instant group explosions" we are seeing now.
At the same time, it would give him a little bit of attrition gameplay, and encourage damage diversification, while staying true (or even moving closer) to the class' style.