What determines what enemies you hit?

Let's say that you're killing a 3+ size mob with TT or BSC, just shooting 1v1. What causes you to hit separate enemies for each hit, but keep hitting the same one on other occasions, assuming they're both in range.

And just to clarify one more thing, say you're fighting BF as a pally. Will Blast give priority to the wraiths that spawned before the BF did and attack them when they come in your range?

I believe spawn order has nothing to do with which ones you hit, priority at first is closest for the first hit, then priority goes to the monsters you hit last, atleast this is true for multimob hits like Shining ray with the exception of heal which almost always goes for the proximity mobs.

I think it's simply something like
foreach (Mob mob in Mobs.Sort(blahblah))
if (mob.InRange(blahblah))
{
mob.Hit
break;
}

which means if two mobs are in the exact same position (and that's almost never true) it's pretty much according to the order they appear in the mob list, which should be either the order they appear on the screen or it's exact opposite.

AoE skills is based on spawn time, ranged skills and melee skills are based on a combo of spawn time and closest to.

AoE skills is based on spawn time, ranged skills and melee skills are based on a combo of spawn time and closest to.

With the furtherest away being the priority. If you use Pierce on a mob of more than 6 it is the 6 furthest from you that get hit while the closest barrels into you .

Power Strike always hits the oldest target in range.

With the furtherest away being the priority. If you use Pierce on a mob of more than 6 it is the 6 furthest from you that get hit while the closest barrels into you .

It's based on the spawn time; it probably happens the way you're describing it because dragon's breath and frostprey are affected by this as well.