I meant the armor peicing weapon mod, not the effect. The mod can't be enabled/disabled while in combat.
I never took the squad effect because the NPCs are rarely shooting at the same thing I am so it puts them at a disadvantage. I always shoot at the heaviest target in the area and let them take care of the closer enemies that I let past.
I was using disruptor ammo with a Locust against the big Cerberus mech. It took 30-60 seconds of firing to wear the shield off.
I never have problems with brutes--just got to stay mobile and warp helps a ton (weakens the armor to further hits). Those Screamers are the ones I always have problems with (seems like they have a lot more armor than brutes are and a whole lot smaller of a target to hit).