I'm building Sniper Assassin right now, and intend to use Archeotech Relic Boltgun (the one with armor piercing "Single Shot" and spreadless "Frag Burst") with "+200% Damage against Shocked, Slowed or Stunned enemies" psalm doctrine. This will cover my needs in crowd control and debuffing (Slow on crit from Physical skill tree). The good thing - "Frag Burst" is a Salvo attack, so you will have 19 Enrage tockens all the time.

The only reason you will want a Sniper rifle is Headshots (preferrably stealth ones), but... Stealth is usless. Why the shot from stealth won't count as a Surprise, it's the most suprising thing I can imagine :) If it will work that way, the Headshot from sniper rifle with "400.0% Damage Bonus for the First Attack in Surprise State" will be the Elite killer.

Yes you are right. The sniper cannot be a fun build to run. It can come close when you use archeotech that has "single-target one-hit kills cause an explosion..." but the gun can't do enough damage to always get a one-hit kill. The  daemonforged with "enemies explode on kill..."  doesn't do enough AoE damage.