The EASIEST method:
Start the lite horde with 5 soldiers on the map Lift, once the game starts, without picking up the fabricator, everyone needs to leave the game excepting the host, then re-join with any class you want, once inside, using the speedrun method, complete the first 23 waves, on the 24 leave 1 enemy alive, then the same players needs to leave one more time the horde, and re join with the soldier class, finish the final wave and the achievement will pop.
Speedrun method:
Starts at 02:07