Aqua Team Murder Force

This achievement is awarded to Alliance players for earning level 30 with all 3 of their faction's Nazjatar followers (Horde version here). The level up rate for followers seems to be about 1 rank per day, plus any bonuses from items like Unusually Wise Hermit Crab. This means that without special items, ranking each follower from 1-30 would take approximately 1 month, making earning this achievement up to a 3-month process.

To rank up with a follower, you must first pick one of the three each day. Doing so will spawn a few follower daily quests around Nazjatar, each of which awards 100 follower xp (with 300 needed for each rank).

I suggest that when a follower gets to level 27/30 that you stop leveling him and move on to the next follower. Then when all 3 followers are above 27 finish leveling them in rotation to 30 for this achievement. The reason is that there are drops that only provide a boost to one (by name) of your 3 followers or to all 3 of them. if you get that drop/special item after he's at 30 already its wasted.

In patch 8.3 this is now account-wide. However it does not grant the achievement for your opposite faction alts. So if you are like me and didn't do anything on your other faction because you thought it would, enjoy 90 days of farming until you get it.