Old Faithful

The geyser will do about 75% of the Naz'tar Tempest Witch's health. My group DPS'ed one Tempest Witch to 50% health, CC'ed it, and then all stacked on top of it so that the geyser would absolutely hit the CC'ed mob. Easy achievement.

Old Faithful is a geyser in Yellowstone national park in the USA.

Just a fair warning.

BE EXTRA CAREFUL. Since we are level 100 now you will one shot her as soon as you engage her. If you do this, there's a BIG chance that the whole instance will bug out, you won't be able to continue. I got the achievement on the 1st waterspout and instantly killed her, the machine to activate next boss wasn't clickable, I waited for a soft reset and it still didn't work, so you will have to either:
1. Wait for another day to finish it
2. Cry yourself to sleep about how you will need to travel all the way there again tomorrow on your boosted character with no ports and no seahorse.
3. Post it on Wowhead and give a heads up to everyone else wanting to do this achievement.

I, personally, went for number 2 and 3. Not exactly in that order. Anyway...

If you let her do the 2 Waterspout phases without killing her you won't bug out, I suggest, if you're a melee fighter like me, to remove your weapon and give her a knuckle sandwich; if you're a caster just poke her with your stick until she does the 2 waterspouts. That way the instance won't bug and you can continue.