Warp specialist is easy if you bring someone who has Warp/have Warp yourself on a Collector mission on higher difficulties, as almost all Collectors on higher difficulties have barriers. Just note that you have to deplete the barriers, not just use Warp on them.
Covert action and Last stand requires the Arrival DLC (Which I'm guessing you have). Covert action should be easy, as all you have to do is avoid fights with any Batariens, which is easy as you just need to not shoot them to avoid most fights with them.
Sadly I have very little for Last stand, as I just sort of hid behind a pillar on the left side of the room until the Heavy Mech in round 3 or so is crazy close, then make a break for the right side of the room to get breathing room to finish off the Mech and remaining enemies(Using abilities like Tactical cloak for Infiltrator or Adrenaline rush for Solider to reduce the amount of times I'd get shot in the back.). This...Barely worked, and I'm sure the Mass effect wiki will have a better strategy.
http://masseffect.wikia.com/wiki/Arrival_(assignment)
