I'm a PS3 player and I've done Completionist for both maps so far and this is my best advice to you.
Do it solo. Even players you know will most likely screw up. I did Point of Contact and Nightfall solo. Another player is bound to screw up in one way or another. You will be much better off by yourself, trust me. You will just be waisting your time with others.
I'm with Death Knight on this. I've got completionist on Nightfall solo twice by running pistols only which avoids all of those impossible "Get 25 sniper kills" when you are on the last hives.
I have confirmed if you run "pistols only" with a group that ALL has pistols only on you can still get gun challenges (which is crap) meaning auto-fail.
If you MUST do it multi-player, I suggest this.
1.) Get a group of 4 people to adhere to strict load outs. No dicking around with relics (everyone should have 0 on).
2.) Level your pistol up to where you can hold two guns on the 1st two hives. Split up the guns evenly. Have two people rock Assault Rifles, 2 with shotguns, 2 with SMG's, have 1 guy run a LMG. Screw sniper rifles. I've had a crew of 4 really good players not get 25 kills in the final areas with snipers....you'll get overrun. When the challenge for guns comes up, feed the people with the correct gun all the ammo you can and use your guns to soften up aliens, but don't kill them. Be smart about it.
3.) Make sure everyone calls out the challenge and hoard flares which are crucial for several challenges (get Scopion with melee, kill 10 aliens before taking damage, etc.). Be smart with your flares....don't just through them because you have two aliens on you.
4.) RUN PORTABLE TURRETS. These make the game so much easier. I don't care how good you are with guns, you'll get less kills EVERY time to a guy with a maxed out portable turret, period.
5.) If you get a gun challenge and complete it, have the people using that gun go buy something else you havent seen yet. You'll never get the same gun challenge twice. For ex, if you get "Get 25 assault kills" and do it, have the people with AR's go swap them for a SMG or Shotgun.
Gun challenges are the ones that will screw you up the most if no one has one and everyone spent all their money setting up turrets before the challenge starts. Everyone runs to get one, 1 goes down, the drill gets annihilated, and no one has any money left for ammo...wipefail.
I Know the ins and outs of getting it. I just can't find good people who co operate. The challenge that gets me the most is find the leper When I play solo. When I am in a group the one that always fails is the 50% accuracy during the last hives. Everyone panics and starts unloading rounds. I just need a team of good people who follow directions