To easily gain SP, you must be able to access the 13th Street portrait, have the Undead Killer, and a lot of tonics. Go to 13th Street and use the warp to the second warp room. Walk out the room, and jump up on the platforms and into the next room. There will be three Red Axe Armors walking back and forth. Use your sub-weapon, and make sure it makes contact with all three Red Axe Armors. Then, equip the Undead Killer and kill the three Red Axe Armors. You will then get 12 SP for each Red Axe Armor.