First, make sure you can access the 13th Street portrait, have the Undead Killer, and have plenty of tonics. Head to 13th Street and use the warp to the second warp room. Exit the room and jump up on the platforms and into the next room. Use your subweapon (Knife, Javelin, Bible etc.) on the three Red Axe Armors, making sure your subweapon makes contact with all three Red Axe Armors. Then, equip the Undead Killer and destroy the three Red Axe Armors. You will receive 12 SP for each Red Axe Armor, giving you a total of 36 SP.
Use the following hint to easily max out Jonathan's sub-weapons. The Doppleganger in the Nest of Evil has 2,000 HP while also having 99 SP (the highest available). Before the fight, unequip all your armor and weapons. Then, enter the room and your enemy will have little or no defense against you. Once inside, quickly re-equip the sub-weapon you wish to build up. Once the Doppleganger is dead, leave the Nest Of Evil and repeat. Note: if you have the Master Ring, the 99 SP is doubled to 198. Two rings will net you 396 SP from one enemy.