Thor, please explain why detecting gani is not a good idea (assuming more ganis are NOT added)

What would take up more CPU?

Triggering one time to all NPCs within a box at the time you slash (which can also be done throughout every frame of the attack gani if desired).

Or having a 0.05 timeout within multiple different NPCs checking the same thing?

maybe only check for that if the player is on the screen?

That will loop through all of the players within the entire level or nearby on the GMAP rather than players on your screen, in terms of serverside there is no screen.
The issue at hand is detecting damage from a single player, though if this was a serverside baddy the inefficiency of this method would be even more serious.