Master of Ceremonies

The boss only spawns at the end of the party crasher event if there is a guest that benefits from the atmosphere that killing the boss gives.

There are 5 different party crasher events that can spawn depending on which guests you have.
If you are missing guests that prefer either of the atmospheres that an event provides then that specific event cannot spawn at all, this can be used to our benefit.

If any of your guests prefer either Relaxing or Exciting you have a chance to get the Master's Forces event.
Only if you have a guest that prefers Relaxing will the boss High Inquisitor Vetar spawn when you are done with the Party Crashers event.

If any of your guests prefer either Decadent or Humble you have a chance to get the Stone Legion event.
Only if you have a guest that prefers Decadent will the boss Drezgruda spawn when you are done with the Party Crashers event.

If any of your guests prefer either Clean or Messy you have a chance to get the Rampaging Dredgers event.
Only if you have a guest that prefers Clean will the boss Sloppy spawn when you are done with the Party Crashers event.

If any of your guests prefer either Formal or Casual you have a chance to get the Lost Souls event.
Only if you have a guest that prefers Formal will the boss Sineater spawn when you are done with the Party Crashers event.

If any of your guests prefer either Safe or Dangerous you have a chance to get the Devourers event.
Only if you have a guest that prefers Safe will the boss Kedu spawn when you are done with the Party Crashers event.

Since you need guests that will allow an event to spawn and allow the following boss to spawn as well, the best setup for this achievement is:
Sika (Clean),
Droman Aliothe (Relaxing)
Polemarch Adrestes (Clean, Formal)
The Countess (Decadent, Relaxing, Formal).

Because none of these guests care if the atmosphere is either Safe or Dangerous, the Devourers event cannot spawn at all, which means you do not need to worry about having a guest that prefers Safe.

The amenities you can use for the event that match with the guests are:
Tubbin's Tea Party
Traditional
Glimpse of the Wild
Stoneborn Reserves

With these amenities you will need to handle these events:
Tea Time: Click on all of the tea pots that are at the table and remove the buff it gives to complete the event, or serve tea for +Formal.
A Quiet Moment: Chat with Picky Stefan by sitting in the chair and click the action bar buttons, can be done during the preparation phase.
Traditional Candles: Clean up 10 melting candles around the court.
Roaring Fires: Auto completes (you can ignore this event if you want) after the time runs out even if you do not put out the fires, you need the Anima-Infused Water to be able to finish the event earlier.
Animal Rescue: Take control of Niya and put Escaped Gormlings back in the boxes and the Flopping Groupers back in the water globe in the center.
Ride on the Wild Side: Ride the Hungry Gorm (it is waiting for you at the western side of the Ember Court, does not appear on the map) into its cage, you will have to take the long way around to avoid getting stopped.

Then you just need the two random events and the Party Crashers followed by the Boss.

Good luck on the achievement!

Alright, thanks to the PTR I have finally got this achievement and understand how it works and why it's hard. I may come back later to add more fancy formatting. First, every full Ember Court will have 10 events. There are two each from the Refreshment, Entertainment, and Decoration selections. Two "random" events, and a two-part assault on the Court. You can do this achievement with any variety of events/guests with the following exceptions/notes:

• 
  
• With the exception of Mortal Reminders, events are completable regardless of whether there is a guest for them although they may not be tracked on the map.
• 
  
• Mortal Reminders requires a guest with Relaxing to spawn the necessary loot for the Stolen Mementos event.
• 
  
• Traditional Decorations requires Anima-Infused Water from Sika to complete the Roaring Fires event.
• 
  
• Atoning Rituals is currently bugged and the Ritual of Absolution is not counting towards the achievement.
• 
  

My personal preference for this achievement is Traditional Decorations (make sure you have the Anima-Infused Water), Tubbins's Tea Party, and Lost Chalice Band. Security does not seem to matter although it may impact the assault. For these selections:

• 
  
• 1 - Complete A Quiet Moment speaking with Stefan, this can be completed during the Court set up.
• 
  
• Drop down to the Muck Pool and attack the roadies there, loot Niko's Flute if it drops
• 
  
• Head up the hill and attack the next group of roadies if you didn't get the flute, otherwise just grab Ansel's Violin
• 
  
• Go across the stage, setting up what you can. Grab Chiu's Lute from near Lord Garridan and kill this group of roadies if you still need Niko's Flute
• 
  
• 2 - Turn in the last of the instruments. Kill more roadies if you have been unlucky.
• 
  
• 3 - Shortly before the Court opens you should get your first random event (Theotar's tea set, one of the rare elites, etc). Complete the event and make sure to use/turn in whatever you acquire.
• 
  
• 4 - On the opposite side of the center area from the band is Simon. Talk to him and act accordingly to guide him through the dance.
• 
  
• 5 - Head to the refreshment stand, grab each tea pot and then the anima-infused water. You don't need to serve anyone tea.
• 
  
• 6/7 - At some point the candles will catch fire. Clean up 10 dripping candles to complete the cleaning event. Throw the water on all the fires to complete the fire event. Having a Guest who prefers a Dangerous Court will mark the water and the fires. Otherwise the ones likely missed are up the Tower across from the Rally Bell, down by the Muck Pool, above Theotar on the wall, and below where the guests gather at the start.
• 
  
• 8 - The second random event will spawn at some point. Complete and turn in as needed.
  
• 9/10 - Party crashers of a random type will assault the Court. Fill the bar to fight them back and then kill the main boss at the Bridge.
  
  The benefit to this set up is the two events that can be completed during set-up (Stefan and the Band) and the speed of some of the other events (Tea is pretty much instant, and the candles and fires are always near each other). Extra time is not required but can be useful depending on the events you're doing as some of them are slower/harder than others. For each event:
  
  
  Spoiler

  
  

  • Tubbins Tea Party
    
  • - Quiet Moment - Speak to Stefan. Use each of the three actions. Can be completed during set-up
    
  • - Tea Time - Use up each of the tea pots. You don't need to actually serve anyone. You can click each one after the other and right-click off the buff.
    
  • Divine Desserts
    
  • - Food Fight - Flip three tables. Then using one of the tables toss food at guests to fill the bar.
    
  • - Made to Order - Use Wicklick the Dredger to serve guests. Unlike the World Quest you must click on each guest you are serving to have the ability light up.
    
  • Mushroom Surprise
    
  • Lower Your Standards - Loot wild fungus from around the Court and turn them in to Stefan. First turn in completes the event, further turn-ins increase Humble. Dog Bone can provide another bonus to Humble when this event is up but doesn't count as another second event.
    
  • Fungi Experiments - Click each of the mushrooms in the room by the Muck Pool. Somewhat dangerous to do all at once. Can be completed during set-up
    
  • Atoning Rituals
    
  • Ritual of Absolution - Currently Bugged Kill the spawning mobs to fill the bar.
    
  • Ritual of Accusation - Match each soul to a sin
    
  • Glimpse of the Wilds
    
  • Animal Rescue - Use Niya to gather animals and return them either to the water or the boxes
    
  • Ride on the Wild Side - Ride the Hungry Gorm around the edge of the Court and into the Cage south of the Stage. (This seemed easier on the PTR, I don't think the Runestags were around for food on live.)
    
  • Lost Chalice Band
    
  • Concert Kick Off - Gather the band's instruments and set up the stage. Can be completed during set-up
    
  • Dance For Love - Teach Simon to Dance
    
  • Traditional
    
  • Traditional Candles - Clean 10 dripping candles
    
  • Roaring Fires - Put out all fires in the Court. Requires Anima-Infused Water. Can be tracked with a guest who prefers a Dangerous Court. You do not need to kill the spawns (although the elite drops a pet)
    
  • Mortal Reminders
    
  • Cartel Ta Justice - Trap Suspicious Waiters and kick them from the party
    
  • Stolen Mementos - Gather the loot the Waiters were stealing
    
  • Mystery Mirrors
    
  • Mystery Mirrors - Open all mirrors at the event and kill the spawns. Does not seem to be trackable. Likely missed mirrors are up the tower and in the room by the Muck Pool.
    
  • Prison Break - Kill prisoners who have escaped the light prisons
    

Good news, everyone! Massive changes to Ambassador's Reserve in 9.1. These were not in the patch notes.

Ambassador's Reserve is not only no longer unique (3), it now stacks to 100. So stock up on 'em up to your heart's content.

Even better, you can now buy them from Lady Ilinca (the vendor who sells the blank invitations). She sells Ambassador's Reserve for 10 rubies each. So you can reeeally stock up on 'em. This makes the Master of Ceremonies achievement much, much easier.

(Also, I've discovered a Plague that will destroy all life on Azeroth. But that's another story.)