This will be obtainable during the second phase of the boss fight against the Frog King. After you’ve destroyed his armor, he’ll shoot projectiles at you that destroy the floor tiles. After that, he’ll hop onto the arena giving you a chance to attack him. Then, he will jump off of the arena and into the water where he will then tilt the arena up in the air.
When the arena is tilted up, he will open his mouth and try to suck you in. To earn this, all you have to do is use your bomb spell to throw an explosive in his open mouth. There will be multiple attempts at this as he will just repeat the same pattern during the rest of the fight.