http://youtu.be/42wkLMjo3Fg
You see where he stands at the start of every wave? Behind that yellow line, whilst the Geth doesn't move any farther forward than that doorway (also don't go past that doorway to the right of extraction either)? Well basically you don't even have to spilt up like they did in that video, you just have to make sure all your teammates are no closer to that stairway than any of the players were in that video, and make sure you're there at the start of every wave, and then just rush down the stairs and kill everything at spawn, and then just stick together and run a circle around the map, it's so easy and fast.
All you need to know is a Reegar is fantastic if you're having trouble with Atlas', it'll take down their shields in about 2 seconds (at level 10 anyway), don't really use snipers apart from the Krysae (it's too close range for snipers) and you can more or less run any class. Play Cerberus too, they're the easiest faction by far. You're not stuck using the same old boring tech classes, you can play Vanguards (well if you're host, Vanguard glitch is still there) and Infiltrators and Soldiers and anything really. It's MUCH funner than boring old FBW, and about 7 minutes faster too. Once you've learned all the places that Turrets can be set up it'll be a walk in the park.
Tips, use a missile if it's the "kill the targets" objective, you know the Atlas is going to spawn at the bottom of the staircase and if you kill it in less than 10 seconds (aka missile it) the next 3 things that spawn will be Nemisis on wave 3, Guardians on wave 6, and Phantoms on wave 10, which is much faster than killing 4 Atlas. Also, don't kill the Atlas on the other objective waves unless you REALLY have to, just keep it at bay until the objectives are done, otherwise it's just going to respawn. And don't be afraid to use missiles, there will be tons of situations where you come across 2 Atlas standing close to each other and, while they can be easy to take out when you're all together, sometimes it's just faster to missile them.