he did exactly what i said, he made an area attack that erased all his enemies, there was no backlash or anything, we were told that the cost was 1000 mp, but we don't know how much mp he have, and he had fought for a long time, used tons of magic, and still had 1000 mp remaining, now i am even more confused that before.
I have checked chapter 45, there was an explanation about the backlash and the stuff he does there (using only 600 mp) that gives him backlash at chapter 46
Now that kind of atk feels like average things he does all the time, i think he got much stronger since them, i still don't understand how his ability works.