Dungeon Defenders II

Flamethrower Towers are the apprentice wizard's first kind of tower. Add the Wizard to your "deck" (meaning equip him as one of your heroes. Switch to him during a build phase, and hold to bring up your towers. Press to select the flamethrower tower, and a to place it. It will deal continuous damage to all enemies in its area of effect. Upgrading with + will increase the damage.

Use a lot of upgraded flamethrower traps at once to unlock this quickly.

Very easy way to obtain this, load the first level, "The Gates of Dragonfall" on private, in each of the 3 lanes, place a Magic Barricade and a Flamethrower Tower behind it. Now, once you get to the last wave (wave 3), destroy the Barricade and Flamethrower on the left side and let the boss through (you can also set up a Barricade just before the Cart so the boss attacks that, giving you a bit more time before he attacks the Cart), and after the boss walks by, replace the Barricade and Tower.

The boss will attack the Cart (or Barricade if you built one) while little Troll Goblin things keep spawning. The Tower will make quick work of those little guys, and you should be able to net this achievement in one game.

You can add two Towers per Barricade, but it isn't really required, up to you.