This trophy is just one of those random easy trophies with no difficulty to it. You will know a turtle when you see it, they are bigger than you, have spikes on their shell and skull and, well, look like turtles. They are very common to find in the caves and when you find it, it will walk towards you slowly and stop to shoot out spikes very often. Just get one of them to follow you until it's in water. When it's in the water, you can drop dynamite if you want to be safe or you can destroy the block beside the water and the water will spread out between the two, giving you room to go beside it and melee kill it.

If you would like a picture of the turtle for reference, see the picture to the right, it's at the bottom of the image standing in the bottom tunnel on its own. The picture is from SteamWorld Wiki.