In Chapter 7, you will reach a point where you must fight a Command Master, 2 Sailors and 2 Pacifistas.
The Sailors, once dead, stay dead, but the Pacifistas spawn infinitely and give 20 SP each. As long as you have 3 accesories and some good skills (up your attack, def and HP skills), you can kill these quickly without too much worry about dying.
The only thing to look out for is the Command Master, especially if you're using the ranged attack in Serious mode. While powerful (and a VERY fast way to kill Pacifistas), it pierces and does AOE damage - make sure he is clear of the Pacifistas and watch out for his attack where he runs right up to you. He can take some punishment but if he dies, the grindfest is over.
If you DO kill the Command Master too soon, don't worry. Finishing the Chapter will still give 500 SP. You can then clear the game, do Chapter 6 again (another 400 SP) and get back to where you were relatively quickly.
This certainly feels a lot faster than repeating the last 2 chapters over and over. In a matter of minutes you can rack up a few hundred SP.