What you're dealing with is basically just the fact that it's a slower payoff than a melee specialization.

Sticking with the cloaking and sniping tactics will payoff.  You'll have to invest a lot into building your rifle up, but eventually most enemies will be one-shot kills unless they have multiple layers of shields.

Of course there is a lot more in the way of hybrid options in ME3 than there was in ME2 so some of it will be up to you and how you play the character.

**Cloak + Shotgun = AMAZING!**  

Right now my load out is:
Mantis V
Wraith V
Shuriken V

Cloak and Operational Mastery are at 6 and Fitness and Disruptor Ammo are at 5.  Everything else is at 1 until I max the others out.