: I've completely reworked this solution after getting both mini and Large crown cheevos! hope this helps you guys.
To get this achievement your going to need to get ready for a grind. Your going to want to be picking up all the footprints while your playing the game and delete all the investigations that dont have 1 silver / 1 gold in them. I had 0 luck getting any large or mini without 1 S/1G.
At the time of this solutions there are 3 weekly quests that rotate ( you can view the rotations here: http://www.monsterhunterworld.com/us/eventquest/
) that you want to be looking for. These quests have a much higher chance to give you crowns since there are multiple monsters in each quest (keep this in mind for further investigations).
Deep Green Blues ( Ancient forest) - Great Jagras / Pukei - Pukei / Tobi - Kadachi / Anjanath / Rathalos
Wildspire Bolero ( WildSpire Waste) - Barroth / Jyuratodus / Rathian / Diablos / Kulu - Ya- Ku
Coral Waltz (Coral Highlands) - Tzitzi - Ya - Ku / Paolumu / Legiana / Pink Rathian
I wouldn't worry about finding these with investigations as the event quests are much easier and you can knock out multiple monsters at once.
Now were down to working with the rest of the monsters. The best way to do this is look for investigations that give you 15 mins for mini and 50 mins for Large crowns. It's best to look for investigations that have 1-2 faints as well as they usually have better luck but the time seems to be a big factor. There are thousands of webpages that post the different specific investigations that you need for crowns. IGNORE ALL THESE
. Crowns are pure RNG and if someone else got it, it doesn't meant your gonna get it. Focus on the times. If you find a 1 faint / 15 min / silver silver gold gold you will most likely get a mini crown. While your looking through the investigations if you can find one that has multiple monsters in the same one with a different time/faint then make sure you test these out. Most likely this will change the monster sizes.
There is a post on TA about crafting max potions and the such to try and change the monster sizes to your liking. To me this method never worked and just wasted time. If you want to do this method it's best with another person so that you can lower the monster, they join and disconnect from the internet and finish the monster on their own to see if the monster is a crown. if it's not then you abandoned the quest and do the method to make it larger. Again I didn't use this method as it felt really a waste of time.
I would really suggest trying to find a group of players that are all searching for crowns as this makes it go way faster as more people are searching for each monster (especially Kirin).
There are many videos online showing the videos of how to measure monsters. The thing I will suggest is to just go in and find how the monsters look for you. After 2-3 fights you should know the "normal" size of monsters. Usually with most monsters when you see a mini your gonna know it's a mini, it just doesn't look right with how small it is. The same goes for Large. when you see a large your gonna say to your self, "wow that thing is huge
When you want to check what crowns you have or sizes the best way is to hit Start -> Tab to Info tab -> Guild Card -> View -> LB and that should show you everthing. This is much easier and cleaner than looking in your hunters notes.
When you get the crown it will show up in your rewards screen like so:
Here are all the sizes for the crowns by yours truely:
I hope this helps and welcome to the grind :)
You will get this achievements in the process: