The time was not on Auron's group's side. When Auron's group arrived at the [Fire Guardian]'s location, they had used more than 20 minutes.
If Auron's group used another 10 minutes to kill the boss, they would consume around 30 minutes to kill one guardian. However, there was more than one guardian in this dungeon. There were four, to be exact, which means they would 120 minutes to kill all four guardians.
On the other hand, the time limit on this dungeon was only 2 hours. Hence, Auron's group would not have enough time to kill the last boss, which was the most important boss.
When the others were attacking the boss, Auron joined the group and approached Smite. He wanted to consult Smite about their current condition.
"Nope," Smite rejected Auron's offer to lead the group back while throwing his spell.