World bosses
A world boss is one enemy with one health pool, shared by everybody on the server, running on a fixed schedule. You are not trying to kill it. You are trying to be responsible for a large enough slice of it, because rewards are tiered by contribution rather than by the killing blow.
How it works
Each event gives you 6 strikes. A strike is a normal simulated fight using your normal build, and the damage it deals is added to the shared pool and recorded against your name. Because the pool is enormous relative to any one player, the boss is a damage-total contest across the whole population rather than a fight you win or lose.
Best practice
Build for total damage over a long fight, not for a fast kill. Nothing about a world boss rewards ending the fight early, and survival is damage: a strike that lasts twice as long contributes roughly twice as much. That inverts the usual priority, and a sustain pet or a defensive potion is often worth more here than an offensive one.
The common mistake
Bringing the burst build that clears dungeons fastest. Burst is optimised for ending fights, and a world boss cannot be ended. The setup that farms best is usually the one that contributes least, and switching pet and potions for the event is worth more than any amount of gear.