CastleStorm

First thing you'll want to do is the campaign until you unlock the "Homing Eagle". Now, make sure to equip it, and select any castle vs. castle level.

From the start, spawn your hero, and attack your own gate as much as you can. After the timer runs out, you can use ballista ammo to damage your own gate more. Once the gate is open, use your ballista to kill all enemies except for one. Once that enemy walks into your base and grabs the flag, arm you homing eagle, fire it and then press A for it to home into the enemy and kill them. Achievement unlocked.