Anteris knew that utilizing his Fire Element Mana would only be useful in neutralizing Regis' Lighting and Fire Element attacks, since Regis was also a Fire Element individual, and was at a similar level to Anteris, Anteris' Fire Element attacks wouldn't do much damage to him.
This was due to the fact that a person's Elemental Affinity was based on which Element they were innate resistant to, and as their Mana Core's grew in power, so too would their own resistance.
Against Regis, Anteris would have to utilize his aura to its fullest extent, solidifying his mana in order to be able to do any real damage to Regis.
Regis, on the other hand, had no such concern, he could rely on his Fire Element to neutralize Anteris own, and his Lightning Element Mana would be more than enough to damage so long as Regis could break through his aura.
