Ninjutsu: which one to actually use
Two of the four pull most of the weight. The other two are situational and easy to misuse.
Mijin. Joe explodes, kills almost everything on screen, and costs him a life. The trick is in the small print: the next life spawns at full health with the ninjutsu slot refilled. So cash Mijin in only when you're already about to die — you've turned a guaranteed death into a clean reset with a free spell attached. It's the safest panic button against any boss you can't out-damage with shurikens.
Karyu is the four-dragon flame sweep. It deletes a room of small enemies and hits bosses harder than anything except Mijin. If you save a slot for "later", save it for Karyu.
Ikazuchi wraps Joe in a lightning shield that eats a handful of hits before fading. Useful when you know a boss is about to land combos on you — Shadow Ninja in the Tokyo disco, Zeed at the end. Outside those moments it's the weakest of the four.
Fushin raises jump height. Only height, despite what some walkthroughs claim about the famous seaport gap in District 7. Worth it on the Chinatown traffic-light platforms. Rarely worth a slot anywhere else.
The Mijin loop
The once-per-life ceiling on ninjutsu has a hole in it. Get to a boss on your last life — burn through your reserve with Mijin if you have to, and watch your score so points don't drop a free one mid-fight. Beat the boss. The instant his head hits the ground, fire Mijin again to kill Joe. Instead of Game Over, the game drops you on the next stage. From there Mijin is yours to spend whenever a life refills.
The trick is fussy: one stray extra life from points and it collapses. Pulled off clean before Stage 8, it's the closest thing the game has to a sandbox mode for the back half.
Beating Zeed, the final boss
The hardest fight in the game starts before you walk into the room. You won't beat Zeed without the Power Pack (the only weapon strong enough to deal damage in the window the chain gives you), and you need at least two Ikazuchi uses in reserve — don't spend them on labyrinth enemies. The Power Pack is in the labyrinth: after exit 20, throw a shuriken spread at the left wall. Lose either piece and you're fighting with weaker tools and a chain that will win.
Once Zeed rises from the floor, he walks toward you swinging the massive clump of hair he uses as a weapon. The opening you want is when he whips it: his belly is exposed, and a sword strike there knocks him back, stunned with a yellow glow. He'll respond by throwing the entire mass of hair at you. It's basically unavoidable, though a fast somersault clears it about seven times in ten.
The chain on Naoko's cage is the second clock running in this fight. Buy a few seconds back by shuriken-ing the small hole halfway up either wall; it stalls the descent without stopping it. Activate Ikazuchi before each Power Pack engagement, then burn the second one when the lightning fades and finish him. Lose the chain race and you still beat the game, just on the bad ending.
Free lives and bonus points
Conveyor belt trick (District 4-2)
The Engine Factory in Detroit is the easiest place in the game to farm lives. At the start of 4-2, fire shurikens at the base of the first conveyor belt — a small Musashi symbol appears. Hop on and ride the belt to the end. You lose the life you're on, but two new ones drop. Repeat until you've banked enough to risk Stage 8.
If you're playing online in the browser and don't want to lean on save states, this is how you walk into the labyrinth with a full bench: two lives per cycle, paid for in time and one death.
30,000 Bonus Points
Finish a level with exactly eleven times more shurikens than lives remaining to grab a 30,000-point bonus. With one life left, that means crossing the goal with eleven shurikens on the counter. With two lives, twenty-two. Extra lives are awarded every 50,000 points, so two cycles of this hand you a free life on top of the cheat itself.
Cheats
Infinite Shurikins
On the Options screen, move the pointer to Shurikins and set the number to 00. Wait about 10 seconds without touching anything. You'll hear a sound, and the "00" flips to an infinity symbol (∞).
Without shurikens, Joe falls back to close-range melee, and most bosses don't let you stay close enough to use it. The infinity flag persists through the whole game once set.