Assassin's Creed: Freedom Cry

You will automatically unlock the blunderbuss at the end of Memory 3 - Laying the First Brick.

The best location for this trophy is during the last mission, Memory 9 - de Fayet's last stand. You will get to a stage in the mission where you will be chasing him through Port-au-Prince. At the same time there will be at least 10 guards in pursuit of you. Simply turn around and hold to aim the blunderbuss into the group of oncoming guards and press . You will most definitely kill at least 5 guards with one shot. If not then simply reload your gun by pressing and try again.