Fair and square: choose a map you're comfortable with, pick a full team and beat 50 waves on the lowest difficulty.
You can do it with level 1 characters, but it's easier if you level up them a bit and equip few useful cards such as those who increase damage. Also don't waste points on fortifications in the beginning since they will most probably be destroyed on the first boss wave, use them to improve your skills by pressing
and the directional pad.
I suggest using at least 1 engineer (repair tool, turrets discount)
We picked the map "Reclaimed", which has a balcony where you can stay covered and shoot enemies, with two people and fortifications/turrets on the sides.
Picking the lowest difficulty allows you to restart from the current wave if you all die. Achievement will unlock anyway as long as you manage to stay in the game til the end and don't get kicked out/rejoin (watch out with gears connectivity issues. If you get kicked out, even if you rejoin instantly on the same wave you won't unlock the achievement)