Originally Posted by Broly

It's shown that 18 was holding back on Vegeta when she completely turned the tables and broke his arm with a single kick.

If Cooler wasn't giving full effort, then why was he constantly attacking Goku and Vegeta? He never gave them a chance to have a breather. He was out to kill them. The fact that he was dealing with 2 Super Saiyans would testify along with this I would say. They're both, Goku and Vegeta, of inferior power compared to Meta-Cooler, but together they pose a threat. And I believe he took this to heart. A good example of what I speak of would be when he was seriously scanning the surface of the battle-site for Goku's position right after he slugged Vegeta. He did not hesitate one tittle in his procedure of combat, and immediately launched ki Goku's way when he found his position.