I know I'm going to sound like I'm on a high horse, but I was the ONLY person (back in 1991) who could beat the Arcade version of Sengoku (the original one) on one quarter, at my local arcade, and I don't know a single player who was able to beat the game at that time (most simply considered it too hard, and if they played, they pumped quarters...)
The game was actually NOT that hard. It just required a LOT of technique AND stalling. Magician Lord was actually much harder (though I beat that game on one credit as well, several times, but I couldn't do it repeatedly, because it required TOO precise play). Besides important tactics like chopping up the enemy samurai's sword as the human, and then IMMEDIATELY moving on TOP of him so he cant kiick you, while punching him and changing direction to KEEP him on top of you (this was critical; you could kill all of the red samurai this way without needing a morph), the trick to this game was knowing the boss patterns, and conserving /recharging your change time. If you knew how to stall, the game stopped being impossible.
I recall that the blue samurai's swords could NOT be broken easily, I think, while the red ones just required you to keep your distance and mash the A button, and when they got close, you would break their sword, and then hug/punch them to death on top of their sprite...I forgot how to handle blues....
As far as the change time issue went,
the game had a "hurry" counter (like TMNT, another game that I routinely beat on 1 quarter), but you could stall it by creep jumping. You did this by pressing jump, and holding the joystick forward while jumping straight up; this made the game think you were moving, while making you go as slow as possible. This was very important, because it gave you extra time to recharge your change time. This strategy was needed on all levels.
Another (easier) strategy was to stall on the areas where the jumping ninjas came out, when your change time was low. I think you could stall about two minutes at these parts, just dodging the jumping ninjas, but I don't remember if you needed 1 or 2 ninjas alive on the screen, in order to not get the death barrels sent at you.
When you learned how to do this, and learned the proper boss strategies, this game suddenly became alot easier.
Last boss was basically impossible to kill without dying, since you HAD to be the human, and I couldn't find a clear defense pattern to avoid his slashes. Basically, mash until he dies, and lose about 3 lives in the process. But if you had enough extra men from precise play, you would win on one credit.
Now, I haven't tried this game on MAME emulators yet, as I simply don't want to deal with this type of stalling when quarters aren't involved like the old days, but I'm sure with MAME's save feature (and memory card saves from other consoles), other strategies could easily be found that were more effective and faster.
Just remember that back in the arcades, you couldn't reload saves, and playing cost money. So experimenting was VERY expensive, so you cared about NOT dying and not messing around, so you don't go broke...
in front of them, retreat half a screen and charge a fireball (hold 
