Gu Fei's Magic Attack Power was terrifyingly high and it could insta-kill many people. In fact, any job class with low HP would have no way of surviving his spells. Right now, however, he was facing three Guardians that had allocated most of their points toward Endurance and were holding three shields with high magic defense. Given all these, it was not surprising for Gu Fei to be unable to insta-kill the three.
As for the other seven players that were also hiding behind the shields, they were essentially unaffected by Gu Fei's spell. This was because the system had merely calculated the damage dealt toward the ones' holding the shields when Descending Wheel of Flames had landed on the enemies.