The penultimate stage of the game is the toughest one on ninpo resources, as you want to start this level full on potions. But here's the thing : as you only have 3 ninpo potions in this stage most of the time, you have to rely on those shiny red drops to avoid lose critical amounts of time in a PB attempt. And I'm not even talking about fights RNG...

Here are the basic data :

• You have 3 ninpo points available. Maybe more with item dupes, we'll get to that later.
• You start the tower with full ninpo potions : 3 big for full recovery, 5 small for one point.
• There is a big ninpo potion on the top of the tower, before starting the demon gauntlet.
• You may still have your rebirth talisman at that point of the run, but want to save it for Fiend Murai. Trust me, that's a good plan.

If we do the math, you have a total of 20 ninpos to use if you burn your entire ninpo stock. With 5 ninpos for each dupe up to 2, that makes 30. You might do even more dupes, but large ninpo potions only give 5 points back at best.

With all that in mind, let's take a look of how much ninpo you actually need, and want, in a "max inazuma" scenario :

• Tower, 1st floor : 2 for each of the 3 waves of red demons, that makes 6.
• Tower, 2nd floor : same as 1st floor, we don't count the extra pill bugs.
• Tower, 3rd floor : 2 to snipe the armored imp.
• Tower, 4th floor : 4 to kill all 4 armored imps.
• Gauntlet, first imp fight : 3, one per imp wave.
• Gauntlet, second imp fight : 3, same as the first one.
• Marbus : 1 to get a full combo opener.
• Emperor, second phase : 2 fire wheels to protect from skulls.

Time for some math again : 6+6+2+4+3+3+1+2 = 27. That means you need 7 drops if you don't get any item dupes, 2 drops with one, and you get 3 extra with 2 dupes.

But after some training, I've made myself a "max economy scenario". The goal is avoid any superflous ninpo use, with only regard to extra risk taken, not potential time losses. Here it is :

• Tower, 1st floor (-2) : 2 for each of the 2 first waves of red demons, and finish the two last ones with UTs. A red demon hit at melee range with the inazuma will usually trigger its faster death animation, slammed on its back, giving you a good timing to finish the two last demons with UTs, one at a time, without burning all essences around at once.
• Tower, 2nd floor (-2) : same as 1st floor, 4 ninpos. There's only one final red demon and plenty of essences from pillbugs, rush onto him to UT is a good way to go.
• Tower, 3rd floor (-1) : One ninpo to weaken the armored imp, and pray to kill him with a very quick gleaming blade.
• Tower, 4th floor (-0) : You want 4 ninpos here, don't even try otherwise.
• Gauntlet, first imp fight (-3) : no ninpo necessary for trash mobs, but very time costly.
• Gauntlet, second imp fight (-3) : same as the first one.
• Marbus (-0) : no save possible.
• Emperor, second phase (-0) : same as Marbus.

Here is the final "eco" score : 4+4+1+4+0+0+1+2 = 16. Great news, we have extra ninpo even without item dupes. But relying on your sword only will be a time loss most of the time, especially on gauntlet trash fights. And I did not count potential missed hits from your inazumas. For red demons in floor 1 and 2, you can manage to keep a good pace with one inazuma and a good sword combo. The armored imp in 3rd floor can also bring you one floor down, but for me, it's a once in a lifetime event. Ominous premonitions, I know...

2021 EDIT : Judging from my extra experience from this level, I've made myself an "optimal" strategy to mix all variables the best I could. Resources and speed of course, but also the possibility a major enemy gets out of inazuma range. I'll use the same system as the "eco" scenario to explain it, minus the parts where you can't negate ninpo use.

• Tower, 1st floor (-2/-0?) : saving 2 ninpos in the same fight is a huge benefit, but depeding of how the second wave of demon dies (where you will use inazuma #3 and #4) and how the third and last one spawns, you may burn all essences in the room to kill the demon #7, leaving you the last one with no quick UT available. As using inazuma on a single demon is basically a no-go, you will have to train this fight to avoid wasting essences. Burning 2 ninpos for 2 demons is acceptable, but that most of the time mean I got at least 2 ninpo drops from the 6 previous enemies. Not impossible, but very unlikely.
• Tower, 2nd floor (-2) : same as 1st floor, 4 ninpos. As the fight usally ends with only one demon and one pillbug left, give the demon a good'ol UT and a 2 or 3 hit combo will deal with the pillbug. That being said, the pillbug can also try his leap attack on Ryu doing his UT, resulting in a final double kill and a faster door opening!
• Tower, 3rd floor (-0) : the plan is simple. Cast your first inazuma as soon as the armored imp and his 2 friends are in range, wait for the 2 adds to spawn and come to you (if the armored imp is far enough, wait for him to jump on you or cast his beam, that's a good hint), and repeat. I got rarely trolled doing that.
• Gauntlet, first imp fight (-0),second imp fight (-0) : Inazuma is deadly on imps, killing them instantly as they spawn if you time it correctly. But depending of how much ninpo you have left at this point of the level, these are also the fights where you will want to sacrifice time to save resources.

That makes an optimal score of 23. Not bad, but that's still 3 points more than what you start with, and as many red drops you'll need to clear the level. Then, how do I deal with the whole situation? Well, I have two methods.

First, to manage my whole ninpo stock during the level, my basic rule is "seven to save". Starting with the "optimal" strategy, I keep in mind the score from each ninpo I saved so far from not spamming inazumas, and the red drops I got from enemies. If that total reach seven, that mean I can finish the level with maximum ninpo consumption using all my potions, assuming I don't get any cheesy dodges from, let's say, 4TH FLOOR ARMORED IMPS GOING UNDERGROUND OR BEHIND A DAMN PILLAR? Sorry, dark memories coming back.

You may think about throwing away your rebirth to get the 3 ninpos you need back, but you need to remember : you'll usually reach ch15 after 90~100 minutes speedrunning earlier stages, and even if he's rather weak and cheesy, Fiend Murai will have no remorse giving you a cheap death and game over if you gambled your rebirth the level before. And you can lose up to 2 minutes on the last 2 stages, from bossfights RNG to skullfish grabs. Staying focused is vital here.

Then, I may have to rely on red drops from skullfishes to get enough ninpo. If so, I'll try to jump attack in front of the entry door of the upper floor, most of the time slaying 4 or 5 skullfishes, and use the door once their death animation is over with essences spawning around. As Ryu can gather essences while crossing doors in this place, I'll waste no time if there is extra ninpo to catch. I use instant jumps on stairs out of this : you really need some good timing to not get grabbed, but it's quite safe with proper training. You'll die quite a lot trying this, but that's worth the time invested.

That's all for this subject, hope those strategy tips help you getting some shiny PBs!