Dungeons 4

In the overworld you can find carts around carrying TNT. Defeat the heroes guarding them and you can take control. You can use them to open shortcuts by destroying rock walls but for this achievement you must murder with them instead.

You can control these carts individually by selecting them with while your army does something else but pressing allows you to swap to the army and vise versa. Move the carts near a group of heroes and they will start to attack it. When it runs out of health after a delay it will explode and should end the pitiful lives of most heroes around it. It's nice to try to also have them close to a defending tower to get some damage in on that too.

Just note that you can only move a cart next to heroes. Clicking right on them won't move the cart. There also is a finite number of these carts in the world so try to focus on getting this out of the way as early as you can but it only took me 2 or 3 carts so it's not hard, just kinda missable.
Also keep in mind this level has an achievement for no minions to get caught in the storm so just keep that in mind while working on this.