Supprisingly, it's not that difficult.
I got to wave 23 BY MYSELF, simply by using a boomshield. (I keep running out of ammo mid fight)
Always collect ammo/weapons when there's about 2 locust left, then run back to your spot and wait for the other two to surface.
I don't have working live at the moment, so I'm stuck hording with myself (would have been nice to have bots)
Whatever map you do it on, find somewhere where there's only one way to get to you, for instance the spawn point on blood drive.
After wave 7 or so, the big whacker locust come, with boomshields. Deploy it, so it blocks the entrance, and as long as you keep near it, you've perfectly safe. (easy to kill advancing locust, and even when crouching behind the boomshield, whacker locust don't hit you.
Regular locust rarely come towards you, and if they do it's easy to shotgun them to the face before they knock it down. As with the sire's, one close range shotgun round will send them to the floor, so you can shoot them whilst they're down.
This is me by myself, by the way, so with 4 others, it should be insanely (see what I did there?)
Easy, provided you always blindfire when there's a grinder about.
Look at some video's on youtube of people doing it, this should give you some help.
Practice by having warzones against insane bots(also use this time to find camping spots) I always play against insane bots anyway, all the other difficulties are way too easy.
sXe