This achievement is based on a daily, you can kill a rift each day.
To kill a rift, you have to kill elementals around it, its health/mana will decrease and it will die
You then have to click on the object which appear after the rift death, take the quest and validate it in a capital
the quest won't appear on the map if you're 80, it's a bug, it's considered as a low lvl quest (?)
if you want to validate the daily on several characters, just put your toons on the same map, even if the character didn't take part in the killing of the rift, he will be able to start the quest by clicking on the object on the floor.
Fire Powered
Zuldrak is great, take the path shown on the picture below, it will take some time, farm mobs, search for gathering materials near it, or watch a movie and check this 3 spots from time to time.