Out of the three, he crashed out the Phantom Illusion skill without any question. The illusory copy might be able to confuse the enemy beast, but how long would it last was a question he was not willing to know the answer to.
The Mirage Projection was a good option, too. Being able to create a large illusion to confuse the enemy so that they could escape unscathed. From the description, Lucas could tell that it was very powerful.
However, Lucas ultimately chose to let the Mirage Worm inherit the Unhindered Stealth skill in the end.
He did not forget that Spot was extremely powerful and could fight above his level due to his transformation skill.
