You can also do it by yourself, just put these mutators on: Endless ammo, Super clip, Strong Melee. Or whatever the names are, but you get the point. Just put the difficulty on Casual and you'll be fine, it might be boring, but it's easy. For Forces of Nature I recommend you use "Aftermath" and for "Fenix Rising" I recommend "Escalation". Both have top-control and have good weapons (shame Escalation doesn't have Boomshot though, but Digger does just fine).
Like I said, it might take a while, but it's easy to do. You don't necessarily have to do all 50 waves at once, you can stop at 22 or something and then return on that wave again when you feel like it, still count towards the achievement (I speak out of personal experience, no worries there) and while doing this, you can upgrade your fortifications as well for the achievement that's tied to it.
The Super Melee is really helpful, you can kill a Berserker in one hit with it.