Lesha, I don't quite understand the meaning of that phrase. Fire Breath only affects the second unit, the one standing behind. And the shield works during attack/defense :)
Besides, the Ifrit and the Phoenix only deal half damage to each other.
He probably meant that both have immunity to fire, so the Ifrit won't deal damage with the shield, but he can deal damage with a direct attack, they won't be able to dodge :D