This is a simple achievement and not very hard to do. Hardest part is finding it. A fully upgraded harpoon strength/storage and rowboat armor would make this a lot easier but is not required.
There are two locations on the map where I found killer whales, One is in Cruz, at (537,340). The other is on the west side of the map in Conttoyor at (120,496). If you have completed the naval fort in that area unlocked, they will be easy to see on the map.
They are faster then the other whales in the game, so it can be harder to hit it with a full power harpoon throw. Although, the health seems rather low on them. Only thing you need to look for when killing them, is after they break the rope they will come up and charge at your boat. Make sure you hit it before it gets to you or you will take a lot of damage. Once it is dead, you will get the "Killer Killer" achievement. Poor Shamu :(