Even though Link could now purchase a Legendary-level spell, he still had to consider all the possibilities before making a final decision.
Right now, he had two options before him.
Firstly, he could choose to run away. An Angel of Light had blessed his soul, all his attributes were now full, plus he had 100 Omni Points, so escaping shouldn't be too hard for him. Once they were safely out of here, they could then go back and gather reinforcements to kill the old turtle.
From the perspective of safety and survival, this would be the best plan because it involved the smallest risk. Except there was one hiccup – Bale and Shade weren't fools. The minute they found out that Link and Herrera had escaped, they wouldn't be foolish enough to stay here and wait for them to come back with reinforcements.