Killer Killer Trophy
Harpoon a killer whale.
Killer whales can be found in a couple locations in the game. You will not be able to see its icon on the map until you liberate the plantation at Tortuga (377,371). The killer whale icon will then appear to the west of Tortuga at location 336, 372. When you arrive at the icon you will need to hold when prompted to begin harpooning. This mini game of sorts is pretty straight forward, simply hold to aim and press to throw the harpoon. It will take numerous harpoons to kill the whale.
Note that the locations of killer whales differs to that of the main game Assassin's Creed IV: Black Flag, as you do not have access to the entire map of that game in this title.
Below is a video by PowerPyx showing how to harpoon a killer whale. Note that the location of this whale will not be the same for you as the video was recorded playing Assassin's Creed IV: Black Flag, not Assassin's Creed: Freedom Cry: