In the Illusory World, Merlin did not know how many times he had tried to combine the Fire-type Wizard Heart and the Thunder-type Wizard Heart. Although he still failed, as Merlin gradually summed up his experiences, he gained a very deep understanding of the two spells.
Therefore, with the passage of time, Merlin's success rate gradually increased even though he was in the Illusory World, and was only "simulating" fusing Wizard Hearts in an illusion.
"I should be able to give it a try now!"
Merlin's Mind Projection in the Illusory World instantly disappear. When he opened his eyes, his entire person felt different.
In the Illusory World, he already lost count of the number of times he "died". Even if it was an illusion and a simulation, Merlin still greatly benefited from it.