Way of the Ninja: Hard is the first stage in Ninja Awakens!.
This stage is beyond difficult. The Flying Ninja Cat has almost three times as much health as Teacher Bun Bun, and has the strength to match. Fast area attack means pawns will not stop it for long. Its only saving grace is that it is a flying enemy, and thus can be warded off with cats like Delinquent Cat.
Battle Items: Sniper Cat
If you have Kotatsu Cat, this stage is ridiculously easy. Just bring Kotatsu Cat, Necromancer Cat, and any cheap damage dealer, such as Dragon Cat. All you have to do is spam Kotatsu and Necromancer. The boss cannot advance, as it will be frozen and weakened. Dragon Cat is just there to make this fight a bit faster.
Macho Cat, Wall Cat, and Dragon Cat, preferably their True Forms (except Dragon). If you have a high enough level for production speed, the boss cannot advance due to too many Cats and Tanks, and the Dragons can do the work from there.
Needed Items: Sniper the Cat and Rich Cat
I don't know if (Angry) Delinquent Cat is needed, if you have it, use it, if not, try without. Also, no cat combo's are needed. Despite what you would think, Onmyoji Cat is a blessing in this stage, barely outranges Flying Ninja and if you keep cat-spamming, Onmyoji never gets hit. Onmyoji weakens the enemy, getting the ninja stuck with the Erasers, which can survive over 4 hits.
When the ninja is a quartre to the way to your base, start spamming Crazed (Macho) Cat. When the ninja is halfway, just get to spamming. Just spam at your leasure. When I tried it, I just spammed the meatshields and spawned Onmyoji and Paris when I wanted and had the money. Try it. Make sure you have Sniper the Cat and Rich Cat, if not, RIP.