It should be still using Magic, unless you added Armor Ignore via Piercing or something. The more Magic Abilities/Special Moves/Abilities a pet knows, the more it'll diversify it's "casting" AI. Give a Fire Beetle nothing but the Inferno ability, and it'll spam the hell out of it. But give it AI and Frenzied Whirlwind as well, and it will cast Inferno less often.