There is a faster way of earning this.
Firstly if you have Novice mode enabled change it to Hardcore in Help & Options.
After that start Beginner Swarm - first level on easy.
Upgrade two of the towers near entrance starting from the left one and place one above the second from the left before the first wave:
In second wave wait until one of the creeps (rest of them should be destroyed) is entering the exit and place one tower in the upper left hexagon:
For blue creep it will take two shots to destroy and tower must be placed when it's half way on the red hexagon.
For light blue one it will take only one shot and tower must be placed when it's in the exit.
If your timing is right u will score x50 multiplayer and after that u have to finish the level to unlock the achievement. If not replay it.