The power of the mental attack he used was at 1(Cultivation Rank) × ((1.6(Mind) + 1(Telepathy Metac)) × 5(Mind Stab/Willpower), which is 13 units. This much power is too much for a normal person with a mind ranging from 0.1 to 1 and a Dao Heart of level 1 to bear.
In the case of this particular enemy, whose mind was at 0.7 and a Dao Heart of level 2, his total mental power without the enhancement of any Metac is 0.7 × 2, which is 1.4. So the power of that mental attack was 9.286 times more than his limit. This made it as if his mind was butter and Loki had stabbed it with a sharp and hot knife.
Even if Loki had let him go after that single attack, his mind would have been terribly injured and would have been in danger of fracturing for good. That's why he felt so much pain and forgot about the fight.
