Siegecraft Commander

Really simple to do in multiplayer with two controllers or a buddy. You and the dummy should play as the humans, either variation is fine. Have your main account (the one getting the achievement) build a line of libraries with some shrines off to the side. After doing this, have your dummy account build four garrisons and four barracks around their keep. Soldiers will start pouring out of the dummies' base. Keep building libraries to buy time and then hit the enemy soldiers with the freeze spell from a shrine. Make sure you have outposts in a gold and gem reserve to be able to cast freeze. Hit ten at once and the achievement is yours. Good luck and happy hunting!

Shout out to CBgamer v3 for the guide

If you wanna see an easy way to set up for this achievement check out my Achievement Walkthrough video at 5:45