I was thinking that the mythical monster can only be created with cells obtained from an event. the cells will have to combine it with some element rock or a relic. at the time of combining the cells with the rest you will have the possibility that a monster type will come out according to the combination, but you can not choose it will be a random process, as well as the attack version of the monster or its defense version or its version of effect. It would give an interesting process to the game apart its attacks could be of combined elements.

@kasterwer A higher level of monster already exist: Nemesis.

I think SP missed a trick by not giving Nemesis monsters more stamina (150) to make them different to the others.