It's still Mokou, though. If fire attacks get reflected, her immortality is still super effective. She can just gradually beat it into submission with a stick, even if she's only doing 1 HP of damage a round.
*FLASH*Wha~For such attacks~!I will just do this-ho~!Burn to nothing!*booom*Immortal 「Fire Bird -Flying Phoenix-」M-Mokou~!