The Ninja Turtle achievement is one that requires a lot of waiting, except in this method! This is the fastest way I found to get the ninja turtle achievement, less than 2 minutes.
How to Obtain Ninja Turtle Achievement
How to Get Ninja Turtle
Create an ocean world, then add a small amount of land. Spawn a turtle and give it the Whirlwind trait. Then, quick spawn humans on the small island.
The whirlwind trait will deal damage to the humans over time. Hold shift and continue summoning humans. Turn on supersonic speed, and should see the kills on the turtle increasing rapidly, after a small amount of time, you’ll get the achievement.
Speeding It Up
There are a few ways to get kill faster. Summoning a plague on the humans and making them mad are two ways to deal extra damage to the humans. Anything that damages the humans will give the turtle kills faster, giving more xp and levels. If you do this, make sure to give the turtle the Immune, Super Health and Regeneration traits so it doesn’t die.