Grandmaster Ninja is the final upgrade in Path 1 for the Ninja Monkey in BTD6. It is a much more powerful version of Bloonjitsu that shoots much faster (specifically 10 shots per second), has much larger range, greater pierce (6 per shuriken instead of 4, 5 per caltrop instead of 3 with Caltrops crosspath) and shoots 8 shurikens per shot instead of 5. It costs $29,750 on Easy, $35,000 on Medium, $37,800 on Hard, and $42,000 on Impoppable.
It is very well advised to spam Shinobi Tactics Ninjas around the Grandmaster Ninja to maximise its bloon shredding power.
When deciding between adding either Path 2 (Distraction and Counter-Espionage) or Path 3 upgrades (Seeking Shuriken and Caltrops), it is best to consider what each of the paths will improve for the Grandmaster Ninja, as each path is more useful in different situations.
If used for popping down MOAB-Class Bloons, it is advised to use Seeking Shuriken, as it will maximise the number of hits onto the bloon, especially against grouped rushes.
If used for good anti-camo defense or for popping down normal Bloons well, it is advised to use the Distraction and Counter-Espionage upgrades. The sheer number of shurikens will make it easy to distract bloons, while Counter-Espionage will help decamoize bloons.
With the Deadly Tranquility Monkey Knowledge, the Grandmaster Ninja throws 9 shurikens further increasing its power.
Note that it still cannot pop Lead Bloons, so it's advised to add some towers that can pop leads and/or D.D.Ts , or even give the Grandmaster Ninja lead-popping power.