Pay to Slay

In order to summon a boss on the Isle of Thunder, you must first collect three [item=94221]'s. Although there are other sources—they're reliably obtainable, once per week, from the rare elites and treasure chests scattered across the island. For a map identifying the location of each bosses' respective ritual stone and a preview of what to expect upon encountering them, take a look at this guide.

Make sure to pickup Champions of the Thunder King prior to summoning; it can be completed weekly.

Caution:
These encounters are intended to be undertaken with the help of up to four additional players. Depending upon gear level, class, and specialization, it may be possible to solo—at least some of them. However, keep in mind that if you die, they'll despawn and you'll have lost your stones.

At the moment, none of these bosses are leashed to their spawn points and will therefore not reset with range. This may allow you to mitigate many of their mechanics through kiting.

You do not get credit from killing any of these if you are not the one tagging it. So when the mob's nameplate greys out, don't get your hopes up for still getting it.

Source: Me helping another group kill it.

While waiting for Horgak the Enslaver to respawn for the dailys, I helped a a group that was killing Electromancer Ju'Le and got the achievement even though though I didn't have the tag. Don't know if there's been a hotfix and it was intentional or if it was just a glitch though.