Story related, cannot be missed.
After your second visit to Mount Sumeru, you need to revisit all locations and fight another round of bosses. The Slime Queen is in the Iron Kingdom.
First, go to Port Maerifa and go to the bar. You will recruit Baozhai again. You can now take her ship back to the Iron Kingdom (run all the way left in Port Maerifa).
Once you arrive in Port Maerifa, use the Ground Splitter technique to smash through the floor near the docks, and follow the sewers/factory through until you get back to where you fought Moloch. Then follow the room out Northwest and up through the smokestacks until you reach the roof where the Slime Queen fight begins (note, just before going up the smokestacks, you need to shoot a red switch with your bow in the room to the left to stop the lava falling on either side of the smokestack - Took me ages to figure that out!!!)
For the fight itself, I suggest switching your entire party out for ranged attackers, as melee attacks will hurt your characters. I used Razmi, Baozhai and Zebei, and just avoided attacking with Ajna.
Halfway through the fight, you will need to build up your Iddhi meter to 5 and use a new move to break through to the boss. After which the fight will resume as normal. Keep cutting away at the bosses health until she's defeated.
Achievement unlocked!