If you unequip the weapons to a character, he/she will start the tactical encounters with the Rusty Peashooter - a very bad handgun. To get this achievement, complete a tactical encounter firing ONLY with the Rusty Peashooter - you can have a character with other weapons in hand, but it's important that you NEVER shoot with these weapons. It's easy to get, specially if you play on easy - some damage enhancers (with cards combination, or equipment) are however really recommended to boost the damage of these rusty handguns.