Rein was going to fight against the Kraken but his lightning magic was not going to be very useful for this battle. That was because the area where the Kraken was staying was surrounded by a special coral that had the properties of absorbing lightning to get strengthened.
Obviously as Rein's lightning was special, it would not be able to use it to strengthen itself and instead would get destroyed from inside. But still due to the salt water and these corals, his main attack method of lightning magic was going to be very limited for this fight.
But Rein was not too worried about it as he still had his light magic and sword. For a green grade transcendental monster, it was more than enough. And if she really wanted, he could just destroy all the lightning absorbing corals before the fight, but that was really not needed.