Mental attacks had some use against Michael, but it was not like his mental defenses were low. On the contrary, Michael had the Legendary Ring Artifact, which strengthened his mental power, and several Soultraits, which enhanced his mind passively.
Last but not least, Wisdom Breaker shattered his mind into countless fragments and refined them before putting them together once again. It was incredible painful and dangerous, but it also drastically strengthened his mental defenses. Therefore, he didn't get knocked out even as one of the Elite Tekur's 3-Star Mind Stab Soultrait, amplified with a Low Elite Class Soul Technique, impacted heavily.
It hurt and was enough to distract him for a quarter of a second, but Michael pinpointed the perpetrator almost instantly and used Cosmic Stride to appear beside him. The Aethyr Claw pierced through the Tekur's skull before he could again release his Soultrait.
