I only played on normal but the last half of the game I never had any SP issues, mostly due to the gear you can buy off Tae. Most of the time I ended up facing the boss at around 70-80% SP left, which makes it a bit trivial. Additionally when you pick up more party members you can swap them around where they will heal SP when benched. Also the reason why the first couple of dungeons may be the most difficult as managing SP actually matters.