In order to unlock this achievement you will need to make straw bundle
x3 Long grass needed - Can find on trees.
This unlocks to craft a wooden rod
x2 Timber - on trees.
You will need a crafting bench
in order to make the spear:
(Note you need to make your bed, a fire pit and a small chest to have this unlocked)
x2 Stone Chunks - Need pebbles smash rocks. x3 Timber - on trees. x2 Rope - Need leaves off bushes.
Finally craft the spear
x1 Wooden Rod - timber on trees, x2 Metal Ingot found from dungeons or chest. x2 Rope - Need leaves off bushes.
You can also find the spears off of orcs, in chest or dungeons. The orcs will be roaming around and fighting them with a weapon such as club will help. If you have monkeys they can help you fight them as well.
Now find an open ocean area on your map; mostly on the sides of the island. You'll see shadow fish roaming.
Equip the spear and hold X
and aim towards the fish and release, this should knock it out and the achievement.