Death
Even though this was an important battle, Theo still experimented with his new ability. It couldn't be helped since this Inverted World was the only ability that was effective against the slime.
Theo had another plan that could allow him to inflict some damage on the slime. Clone Theo stomped the ground with all his might, releasing a surge of wind pressure to the top of the slime.
'Ascension Step, Tenth Step!' Theo even used the Mirage World to imbue the Wind Power, making it look like the Wind Emperor himself was the one that used this technique.
The torrential wind was so violent that it looked like it could penetrate the ground easily. But for the slime, this was something easy to handle since he had both Dissolve and Devouring Power.
The wind was disrupted, losing some of its energy. However, Theo persisted in it and pushed through.