The powerful tsunami of fire was dangerous, but it had taken the snake-like a couple of seconds to unleash it. That meant he couldn't do it without preparation. Now that they knew that, Zach's familiars and the Nilma chief just needed to watch out for any tells that he was using any powerful moves and react accordingly.
He was stronger than them. There was no doubt about it and no shame in admitting it. But he wasn't invincible.
The difficult part would be stopping him from fleeing once they managed to injure him. But that was a matter for then. Right now, their priority was enduring the heat and getting close to the fiery snake.
Fortunately, they had Zach's barriers and Mandra's magic to keep the intruder's fire and heat in check while working through his defense.