Here is how I unlocked this;
We chose Aftermath, and built the main base that is behind the Silverback. This has 3 'funnel-points', so with a decent team of 5 is easily covered.
Mutators do not void the achievement.
On Casual with Super-reload, well, it's just super-easy. Even if you are not the best player, get behind cover and play cautiously!
Build the turrent to your left upto a Fire-Turret, half the Locust on Casual do not get past it. There is also a Command Centre on the left side as well. Throw a load of wire down and you cannot fail.
We also tried the 'Force Multiplier' (all as LE Dom/Baird), we spawned as all Marcus and we failed the first wave on purpose to character reset - we all then restarted and it popped for us all after wave 20.
If you go as Baird LE you can easily work on your 'Witty Repairtee' progress as well.
EDIT;
8th July 2013
Just did this with 2 people, so if you cannot get 5 this will help - did it in just under an hour.
Start - Casual, Super Reload
From spawn on Aftermath, head to the opposite tower.
Buy that base, buy the wire downstairs, and the wire upstairs.
First few waves you will earn $$$, put that in wire repair, as well as frags.
The main strategy is to have the player who needs it tucked at the back near the Command Centre, ideally the player who does not covers the main staircase and the front.
When you can, buy the base at Frags.
This gives you wire, but more importantly - the Turret at the bottom of your stairs. Make this a Fire Turret as fast as you can, and keep that for 20 waves. This does SO MUCH KILLING for you.
Every wave, search/buy Frags, tag downstairs, keep Turret going. On 7/17 start using Mortars. With 2 players, by 17 you can upgrade to Hammers, call them out and play smart.
We did it with the two worst Boss Locust (Brumak & Corpsers), so good luck!