Try to aim for one or two people right now dont worry about getting everyone it will only make it hard to pull of since everyone will be plunging on you.
As for to actually pull it off you have to wait till the debuff is on them then hit one of the debuff targets (obvious I kno but I'm getting to it) you want to delay by a second or a server tick if somehow you can track that because for some reason even though the debuff looks on the LB will snap shot the instant you click it and if the debuff is still being place (floating above the enemy) it will register that its not even on the person and you will get the you did you LB dmg 20k. Also there is a range of effective activation. when you hit your target anyone around you that has the debuff will also take dmg its not the dash but the end circle that procs the dmg
I'm sorry if its hard to read or understand LMK if you have anymore questions