Experimentation

Medusa spat corrosive fluid from its mouth, but William had planned for this as well. When its mouth opened to spit, a single earth pillar raised from the ground at a diagonal, shoving itself down the snake's throat while poison corroded the top of the pillar. 

William controlled the corroded part to detach from the rest, then continued forcing the earth pillar down the snake's throat until it was unable to even spit corrosive fluid.

"Alright you danger noodle, stay still while I test a few things out." William said as he called Mana of the Fire Element to his fingertips.

Instead of forming the usual fireball or giant fireball, William elongated the Fire Mana until it had created a long flaming javelin. He threw the flame javelin with great speed at the scaly hide of Medusa, the System Message notifying him of the creation of another new spell.