Like most Flame Flans, Infernos absorb fire and resist physical attacks, and there is a Breaksight chance when dodging a lunging Inferno (Killsight on its last legs), and then hitting them right after. Inferno enemies are even more resistant, making them even more difficult.
Infernos are more challenging than Flame Flans. At lower levels, even magic will take several hits to kill them and will take a heavy toll on the party's MP. Instead, using physical damage cadets like Cinque or Jack to damage them is a faster and more resource-prudent strategy, especially Cinque, whose ability to knock the Inferno backwards can prevent it from dealing more damage. Additionally, ranged cadets like King and Cater can run up to Inferno, and once it lunges, dodge and hit a Breaksight.