That´s an interesting strategy, but risky for a BLM to deplete his MP so early (when Knight spawns), cause if something goes wrong, you could stand there long time with no Mana, and watching the Knight run towards your Partymember...
could fit in when you are missing the timing and the Knight spawns while you are Firespamming, but i usually prefer being full of Mana when this happens... it´s more on the safe side, in my opinion...