Load Memory 4 called
"A Ship of His Own" and play the Firecrackers Tutorial ( 15-30sec)
Dont jump down to kill the Tutorial Guards, stay at the Roof and go forward. On the left Side stays a Guard at the Roof, kill him and go in the direction to the powder Keg (Symbol)
If you above the powder Keg, switch to Firecrackers and throw it down. Wait until all 5 investigate the Firecracker then Shoot with the Blunderbluss the powder Keg.
Reload the Checkpoint if you failed. This is one of the easiest and repeatable Way for this Achievement.