Quarian Marksman combined with the Adas Anti-synthetic Rifle is cheat mode bad assery.
http://kalence.drupalgardens.com/me3-builder#2A!1501545!!53251364!F9.AG
The important bits are weapons damage upgrade whenever possible, and DON'T take the duration bump for Marksman. You need the magazine extension on the rifle, as well.
The Adas + QM is glitched. The RoF doesn't change when you turn on Marksman until you left off the trigger and start shooting again, which sucks. It also doesn't change the RoF when Marksman runs out until you left off the trigger, which is awesome.
So, hit Marksman and just hold the trigger down. Marksman will wear off and start its cool down before the magazine is empty. It will be ready for reset about the time you run dry. Left off the trigger, hit Marksman again (which automatically refills your magazine as well) and you're right back to increased RoF without the need to reload.
The Adas applies ammo very well, staggers, does a bit of AoE, and really the only downside is the slow RoF, which this fixes. Phantoms, hunters, etc. will stay stun locked. It's a really nasty build.