This Achievement can easily be obtained in the tutorial. After a few tasks, crates will drop into the room inside one crate you'll find a Minifun. Lick it with
and you'll equip it. There will be a lot of Zombies to kill but you need to be careful with your aim as ammo is limited. You need to hold down
to fire and it takes a couple seconds to charge the Minifun.
Achievement at 11:17