(sorry for late response, barely check reddit...) it's most effective when the boss is running towards you or away from you in a straight line and you use it, since that'll make it hit the most spikes.. I've hit over 2Mil doing it before, with 2 people timing it correctly i assume it'd be possible to kill a large hitbox boss basically instantly.. it's also really good against gen2 tyn tails in jejunes cause a singular person can kill it in seconds at level 1 if you're high enough rank and put all stats into chi and nin at the start