The sizes of monster are variable, and every time you kill or capture one, its size will be recorded in your Hunter's Notes. Killing monsters on the extreme ends of that monster's size range will earn you a golden Mini Crown (near the smallest end) or a golden Giant Crown (at the largest end). The size of a monster is random, but it should be noted that assignments and optional quests usually do not have monsters who reach the size thresholds required for golden crowns, so if you want to get crowns, attempt investigations, where the size of monsters will vary more.
Some event quests are much more generous, as the event quest Egg Lovers United always features a giant Kulu-Ya-Ku. Failing that, there are several event quests - one for each locale - that features monsters native to that area. These monsters have noteably lower health than usual, and their size can vary into the golden territory, making them an ideal way to establish some crowns when you need a great deal of them. Of course, you have to kill all the monsters listed to complete the quest, and 20-30 minute runs can get tedious, so only bother with these if you need many crowns, to ensure the best outcomes. These event quests, their locales, and the monsters you can hunt are as follows:
|Deep Green Blue||Ancient Forest||Great Jagras, Pukei-Pukei, Tobi-Kadachi, Anjanath, Rathalos|
|Wildspire Bolero||Wildspire Waste||Kulu-Ya-Ku, Barroth, Jyuratodus, Rathian, Diablos|
|Coral Waltz||Coral Highlands||Tzitzi-Ya-Ku, Paolumu, Pink Rathian, Legiana|
|Effluvial Opera||Rotten Vale||Great Girros, Radobaan, Odogaron, Bazelgeuse|
|Rock N' Roll Recess||Elder's Recess||Dodogama, Uragaan, Lavasioth, Azure Rathalos|
This isn't a perfect solution, either, as these event quests are frequently not offered on your quest board (like most events, they rotate out regularly), and without a powerful hunter with different loadouts and weapons available, it can be quite a hassle to hunt such a wide variety of monsters on your own. On top of that, this won't get you any Elder Dragons or Black Diablos.
Note that this trophy/achievement says "for almost every monster" - don't get too excited, there's no real shortcut here. Excluded from this list are Zorah Magdaros and Xeno'Jiiva, neither of which vary in size. Every other monster, however, does, meaning you'll need twenty-eight golden Giant Crowns to earn this trophy. This is easily the biggest grind in the game, along with "Miniature Crown Master".
Also keep in mind that silver Giant Crowns do not count towards this trophy/achievement, even though earning a silver Giant Crown will unlock "Giant Crown".