King's Ransom

The hidden assassin is princess ky'veza, she is located at the far end of the bridge on the left side (after the boss platform). It was unclear how we revealed her, but when we did, we just had to talk to her. She will then "assist" you in the fight by throwing her famous daggers all over the place. Overall it was very easy.

Shoutout to kotsáni-illidan for figuring it out in less than 5 minutes

Of Mice and Manaforges
During the phase where you need to run up to the boss again, there will spawn small mice that you catch by simply moving over them. Once you've done so you get a debuff. You need to play all three intermissions as the number of raid people are split between the phases. So if you're 9 raiders it'll be 3/3/3. They may spawn in the beginning of the corridor when you're in the last part of the corridor.

Time to Vote! Cute or Scary?
Before you pull the boss, target it and either everyone writes /cuddle or /cower. Cowering the boss will make the boss do +15% damage and cuddling the boss will make the boss take -15% less damage.

Mother of All Tantrums
In the four corners of the boss room there's small pads. Click on them during the fight and kill all four ads to get the achievement.

Cheat Meal
During the fight you need to keep one [npc=241923] alive. In the intermission, once the boss reaches 100 energy, you need to drag the [npc=241923] into the blue beam in the middle of the room. Continue progressing the fight without killing the add just yet. In the last phase of the fight the boss will spawn small [npc=242589] adds. Kite the [npc=241923] over these small adds and it'll transform into a Void Forged Echo. You can now kill the add before you kill the boss and you'll get the achievement.

I See... Absolutely Nothing
Outside the boss arena there's a few tents with some boxes next to them. On top of one of the boxes is a blindfold. Everyone in the raid has to pick this item up, and during the boss fight everyone has to click the extra action button and survive for one full minute. The easiest way is to click it at the beginning, and everyone uses it at the same time. After one minute has passed and everyone can see again, you're free to kill the boss and get the achievement.

Breaking the Fourth Wall
Pretty straight forward achievement. Line the walls up so that there's four walls stacked, and then make sure you break the fourth wall 18 times. It doesn't have to be in a row, but make sure you don't accidentally break too many walls in a line, so you make sure to get at least one fourth wall every break-session.

King's Ransom
Walk past the platform that teleports you to the boss, and continue until you see a black swirlie on the edge of the platform. Move closer to it and [npc=218425] will appear. Talk to her and follow the dialogue. This will make her appear in the boss fight. Simply kill the boss like normal to get the achievement. You do need to dodge her classic knife-attack spell.

Defying Gravity
Simply make everyone get lifted up by the [spell=1243577] in Phase 1 during the fight, and you'll get this achievement. Rotate people in and out with the debuff. Very simple and straight forward achievement!

Couple notes
- Ky'veza just appears when you run to her spot (on that lower-right corner).

- The whole raid needs to talk to her, the boss will have a debuff saying something on the lines of "he's being hunted"
EDIT: seems actually only 1 person needs to do this, as a later random ordinary non-achy pug run on heroic, someone decided to talk to her without telling us (thanks for that dude...), and she just popped up -.-

- One of our members swapped back to dps (after healing for fract), and had spoken to kyveza already... and did so again on their dps char when swapped over.
This was probably unnecessary, but we did it anyway to be sure

- Kyveza spawns almost immediately when boss is pulled, with her dagger lines everywhere.
Obviously if this is not the case for you, then someone hasn't spoken to her, and you can easily jump off and try again

- The dagger lines won't 1shot you, but they can hurt a little bit... they appear throughout the fight, and can be sidestepped as usual alongside all the usual boss mechanics as needed

- If you wipe, you'll need someone to talk to her again to activate her for the following attempt.
On the plus side, if someone had spoken to her without your knowing/consent, on a non-achy kill, then you can easily clear this by just wiping and not talking to her again