The flame wall Ross created wasn't meant to attack, but to act as a cover for him to escape the encirclement.
He did not have time to worry about the others who would be caught up in the flames' path.
Once the flame wall spread out to cover his figure from the approaching void creatures, he ran towards it himself.
Since the ability was powered by his mana, he was immune to the flames.
Furthermore, as a fire user, he inherently develops resistance to high heat. With this, he disappeared into the flame wall that drew closer and closer to the void creatures.
Naturally, the creatures weren't going to blaze right through the wall of flames. They promptly evaded the flame wall once it got close to them.
Some flew higher, others backed away and the rest unleashed their own attacks that clashed with the flame wall.
BOOOM!
