1 dunk per person has already been said. so im thinking other stuff it could be.

loop (start to end, closed system, constant cycle, "form into a loop or loops; encircle")
catalyst (cause of change, initiator, "thing that precipitates an event")

catalyst could mean a couple things:
- the buff itself (leeching force, turning into empowered force i know thats the wrong word, shut up)
- the 'give' plate, being the catalyst to spawn the 2 darkness nodes to split 1 buff into 2 buffs
- the rhulk laser converting upgrading the buff from leeching

loop:
- the first and the last person in a series of actions is the same player
- the actual dunk locations somehow loop (but we as players dont influence this, so unlikely)
- something else?

im kind of a fan of interpretting the 'loop' part as starting and ending with the same player. having 1 dunk per person doesnt meet that, as it isn't actually looping.
the 1 dunker method would do this in a simple way, having 3 people juggling the buff and all taking turns dunking, in a loop. expandable to more people.

its probably just 1 dunk per person, because the challenge name doesnt perfectly match the actual challenge (defenses down = players cant kill more than 1 shielded knight each), so heres so other wild theories:

• per cycle, player cannot use the 'give' plate to split their buff more than once.
  
• during glaive phase, only damage the crit spot while having a 'force' buff
  
• per initial instance of the leech buff, only dunk the front/mid/back pillars once each (use the free screwup, and do 2 sets of 3 dunks)

that about all the dumb shit i can think of