A good counter-intuitive trick is to not use all of your MP on Fire I casts, leave your MP at or just below 1000 MP and start Blizzard III then into Thunder II then from there you should go right back into Fire III and Fire I spam. I don't main BLM but I've downed up to T5 and all EX primals on it using this and I do better DPS than when I would deplete my MP.