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.

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.

Thanks for the post and effort! Please keep sharing more such blog. Now I saved it to my bookmarks so that I can keep me in touch with you

Agreed. The whole system in the game relies heaviy on fast hit, better with AoE.

1. How to keep enrage high: enrage on crit hit 

(enrage on kill , enrage on taken hit is not effective enough. In fact, almost everything that triggers on kill would not be good in -99% levels, because to activate them you need to kill enemy first)

2. How to apply debuffs, especially vulnerability 

xx debuff on hit/crit hit, vulnerability on apply debuff

3. Also, combined with the Ordinatus psalm, fast hit means you can strip down the damage reduction of Boss efficiently.

In brief words:  You wanna hit them hard, you gonna hit them fast.

Heat aura/shock exlode aura are strong because of these mechanisms.

Single target, slow hitting weapons are weak, also because of this. (Bad news for sniper and plasma gun lovers)

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I think it's not realistic to hope the dev guys would rework the whole system.

Still, some improvements could be possible without large amount of work.

My suggestions

1. Add passive skills to give single target attack vulnerability debuffs. 

Add some passive skills, once learned, make each hit of single target skills bring several stacks of vulnerability on the enemy.

And this buff could be set according to the CD of skills: slow single target skills apply more vulnerability. 

2. Add some passive skills to specifically reduce the CD of single target skills.

Something the devs already experimented in previous seasons and new classes:

3.  Specific relic enchant to make single target attacks become small AoE.

In S5 there're such weapons and I think it is successful.

4. Some buff skills to directly get enrage token without hit/kill.

Yes, the sisters faith skills. But crusaders and assassins don't have these skills yet.