World First Kill.

Yo, so i would love to add some kind of "World First Kill" thing on some of my bosses, does anyone have any suggestions on how to do that? I'm aware that i can just add a command upon death to everyone on the boss's threat table, but what should that command be? What plugin would you suggest that has that command?
I can't rely on me being online to see people kill the boss, and i wont rely on them taking a screenshot with a time either. Help is very appriciated <3