The blue-skin Slaad was a brute. He would use his physical prowess to his fullest to attack his opponents.
This monster wasn't that attuned to the chaos element. So there were no surprises behind his attacks. However, due to the lack of chaos, he also had a clear state of mind.
This blue-skin monster could plan his attacks well, hide counterattacks in his defensive actions, and use his mana-enhanced strength to his advantage.
Mia also had dagger arts for close ranger opponents. She knew she couldn't run away from the monster this time to take advantage of her ranger class. That's because there was no assurance that the blue-skin Slaad would allow her to kite him. The Slaadi were simply too unpredictable, attuned to chaos or not.
If the blue-skin Slaad teams up with the green-skin one, things would become twice as dangerous for Eren. Thus, she decided to stay and fight.