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Try Helena's Chapter 3. Try Chris's chapters 2-3 and 2-4. Try Leon's Chapter 4. Try Ada's Chapter 2. Try running through Jake's Chapter 4 as quick as possible.
In this chapter, you can find 2,000 SP in a coffin near where you talk about Simmons and Ada. There are also 5,000 SP in a coffin near the spike trap. Kill the zombies you encounter for extra SP. When you fall into the next area after pulling the levers, let the game save, then quit the level and repeat as needed. For extra SP, try shooting the zombies in the head to make them Bloodshots, which are worth more. These chapters have lots of enemies that drop high amounts of SP. Each Ogroman enemy (the giant-like creatures with red organs on their backs) drops 4,000 SP. There are two Ogroman enemies in 2-3 avoid using the third AA gun to be able to get the SP from the second Ogroman. In 2-4, there are several Napad enemies (the ones with hard armor shells you need to break) worth 1,500 SP each and several Strelat enemies (the lizards that spit spines at you) worth 1,000 SP each. Try using the grenade launcher to kill all the Napads in the foyer to get about 10,000 in a very short time. In this chapter, you're in a race against Chris. If you're quick, you can rack up tons of SP in a short amount of time. Beating Chris to the elevator nets you 2,000 SP. After, when you're hacking the locks, use the shotgun to take down the drones effectively. If you do well, you'll beat Chris and win 14,000 SP. After the Simmons cutscene, the game will save and you can repeat this process for more SP. A good deal of SP is scattered through the entire chapter. Napads are worth 1,500 SP each, while Whoppers (the enormous fat zombies) are worth 2,500 each and Shriekers (the zombies with the large red growth on their chests) are worth 1,000 each. There are also lots of SP drops distributed in the various boxes, including a 4,000 SP drop in the treasure chest after the first Napad. Level 2-3 is especially lucrative killing the enemies in the underground lab can earn you about 25,000 SP in less than ten minutes. Jake has one of the easiest opportunities to farm SP in the entire game. The shopping district part of Chapter 4 has no enemies and two boxes containing a total of 7,000 SP. This section can be run through very quickly, so if you're willing to repeat it over and over again, you can earn tons of SP in a short time.