EDIT *From iVilux in the comments - "You need to die or complete the mission in order to unlock the achievement. If you have the challenge but no achievement, just quickly get into a game and die."So I spent all day trying to work this one out and I believe I have the solution.
The first thing that you want to do is gain access to Morgan's Suite on the top floor. There are a few ways to do this but the method I chose was to do the "On The House" Opportunity. By following this opportunity through to completion, it will not only take you directly to the room you need to be in, but will also clear the bottom floor of all staff allowing you to freely move around (It will also give you the perfect opportunity to push Morgan off the balcony and complete the "I Believe I Can Fly" challenge which is part of the Drop the Bass achievement).
**Posted by Calex dEUS in the comments - "Probably obvious but you can get the 'hold my hair' challenge here too when he inspects the toilet."Now once you are in the suite, you want to look around the room for small, golden elephant statues. I counted 15 on the bottom floor and 3 on the top floor for a total of 18, but I could have miscounted. Feel free to correct me if you count differently. I don't know exactly where all of the locations are but all of them were on bookshelves for me, both up high and on lower shelves. The three upstairs are all in the same room, the one with the heart made of flowers on the bed. This is where all of the staff seem to go when they leave the downstairs area and for me, one of them could see through my disguise so I ended up shooting them all.
There are also a few silver elephants. I don't think these are required but I shot any of them I saw anyway. You should hear an elephant sound after each one you shoot. The problem is that you won't hear a distinctive sound after you have shot them all so you will have to keep count and check each room thoroughly.
When you think you have shot them all, head downstairs to the main lobby. When you get to the bottom of the stairs, you should unlock the "I Will Cry If I Want To" challenge and the achievement should unlock. If you don't unlock the "I Will Cry If I Want To" challenge, then you haven't shot all of the elephants. For me, the achievement didn't unlock until I quit to the main menu so try that if it doesn't pop straight away.
This is an Easter egg by the way so if you want to see what exactly it does. Go outside the main entrance to where the two large elephant statues are and look at their faces. You will also hear unique dialogue from the NPCs but I won't ruin it for you.
If you prefer a video guide then check out the video below which was provided by Maka91.
All credit for the video goes to Maka91.
This is my first guide so please leave some feedback and tell me if anything needs to be added. I will of course credit you in the solution.