Rein was struggling against this monster mainly because his ice and fire elemental attacks were not damaging it at all. But while fighting this monster and analysing it with his eye of Truth, he did manage to find a few weaknesses that this monster had.
He then decided to exploit one such weakness and started to make preparations for that. Rein baited the monster by showing it a very obvious large opening and made it to attack his with full power. A regular human would find this obvious opening suspicious but that was not the case for this monster.
By attacking Rein with all its might, the monster left a big gap in its defence and that was exactly what Rein wanted. So he immediately used his short teleportation ability to dodge the punch that this monster threw toward him.
