copy pasta incoming (though it seems kinda redundant)
I Beat Path of the Master Ninja and All I Got Was This Stupid Trophy.
Seriously; you don't unlock anything for beating PotMN. So lame. Anyway, not much really changes between the difficulties in this game. There aren't any more enemies than any other difficulties, they're just a higher grade (usually) and they do WAY more damage. One throw on MN = instant death, generally. The bosses are probably the hardest part, none of the levels were that hard.
To give an idea of how reduced the enemy count is in this game, between all three of my playthroughs and 5 or 6 team missions, I have around 3900 kills between all of my weapons. A full run of Master Ninja in NG2 is around 4200 kills.
On a side note, #1 on chapter challenge PotMN for chapter 1
Eat it Faiz82!
edit: Actually once you beat master ninja the silver frame around your health bar becomes gold. Kinda neat, I guess.