The art of bloonjitsu allows Ninjas to throw 5 shurikens at once!
Bloonjitsu is the 4th upgrade on Path 1 for the Ninja Monkey in BTD6. This upgrade allows the Bloonjitsu Master to throw 5 shurikens at once instead of 2. However, unlike BTD5, it is not possible to crosspath a Bloonjitsu with both Distraction and Seeking Shuriken.
Just like in BTD5, it costs $2340 on Easy, $2750 on Medium, $2970 on Hard, and $3300 on Impoppable.
Generally, the Path 3 crosspaths, particularly with the Seeking Shuriken upgrade, is better as it allows the Bloonjitsu to make use of its fairly large pierce, relatively high attack speed, and relatively high numbers of projectiles.
This becomes more relevant when the Ninja is Alchemist buffed as its max pierce becomes a whopping 35 (across 5 projectiles), and each shuriken will deal effectively double damage!
With Seeking Shurikens, the Bloonjitsu can also have Caltrops for slightly more overall popping power via regular caltrop attacks onto nearby track. Caltrops placed on the map even keep their Alchemist buffs even if the brew wears off after placing the caltrops.
Arguably, Distraction is more useful lategame because it falls off from being reliable DPS later on, although Shinobi Tactics spam often deals with that aspect just as effectively as one Bloonjitsu with Path 2 upgrade crosspaths.
Bloonjitsu Ninjas are among the best candidates for Berserker Brew and Stronger Stimulant, even after the Version 6.0 Alchemist nerf. This is because they do not shoot so fast as to expire potions so early but at the same time shoots many projectiles at once with good overall pierce and damage. As a result, it is recommended that Bloonjitsus are not upgraded to the Grandmaster Ninja, and are not in range of any Shinobi Tactics ninjas, as these will cause the ninja to use up the buff quicker, though either are not bad options with Alchemist brews even with fast expiry.
In fact, a 4-0-2 Ninja + 4-x-x Stronger Stimulant can solo up to Rounds 52 with almost full ease on almost all maps with decent range for the Ninja.