Excellent premise and for a couple levels, I loved it. Unfortunately, it has some loss conditions that don't just seem unfair (which can be taken as a challenge) but seem to be mean-spirited as well. For example: enemy ninjas kill you on contact. You are not warned of this in the description and their random movements make it difficult to attempt any strategy. Anything other than "rush their front" is just as likely to kill you by their random pathfinding as it is to leave you with no target at all.
Another issue: spikes do nothing to harm enemy ninjas and only slightly discourage them from occupying that tile, while they kill you instantly. I had one level generate a spike tile right in front of me with a shotgun ninja in the center tile. His immediate move was to jump forward onto the spikes(!) and fire at me, so they clearly don't serve as a mutual barrier.
Last issue: both shuriken and shotgun ninjas can kill you AFTER you win a level. If you land the first hit but they fired before dying, the shot will land just after your "victory" message and force you back to the beginning anyway. That's just mean. It's not a challenge or an annoyance, it's a buzzkill of the highest order.
So in summary, you always start in the same place but the enemy location and weapon is random, there may or may not be tiles that only serve to kill you and not the enemy, and two out of three of the enemy weapons can kill you even if you win. Oh, and you have to kill every attacker the same way (from the front, right in their line of fire) because you can't turn and their touch is apparently lethal.
Great idea, but please fix these issues so we can enjoy it properly.