Spigot plugin: one player gets hurt, all the others - program?

Hei,

So you should know BastiGHG you probably know what I mean. I want to play some Minecraft Challanges with friends and basically the whole plugin is ready.
The problem is that I never had to "damage" a player retrospectively…

So it should work like this:

A player gets damage → All others get the same damage

I would be very grateful for an answer ^^

Via event + loop?