The quickest and easiest way to get this achievement is to buy Turncoat Level 1 for 3512 XP at the Heroes Guild. Despite the description you do not need to be evil to use this spell. My alignment was +65 towards good and I could use it just fine.
Now go to Greatwood and find two Beetles nearby each other. Select Turncoat by holding the Right trigger and pressing B to cycle through your available spells. Hold the Left Trigger to target lock one of the Beetles and hold B to cast Turncoat. Once it kills it's former ally the achievement is yours.
If the other Beetle kills the one you have cast Turncoat on press Start > Load Game > Checkpoint and you will be back at the entrance to Greatwood.