Yep. There is always a boss who has everything :). Last night. I faced Smogger. I got the Pixie Queen pet, ANd My friend got a thunder mastery amulet. Try- its in mrleybone, in ... Iron Works. Try, nd tell me. Go
The boss named Luska drops all of the Mastery Amulets which is basically halfway through the dungeon of Waterworks. Through my 24 times of defeating him I've gained the Death and Myth Mastery Amulets from him. Just remember It's a very small chance of obtaining a one, since it's a crown item. Even so if you want a specific Mastery Amulet (like the Life Mastery) and if you don't have the time and patience, maybe you could consider on buying one with crowns, like I did. If you don't then good luck farming.
sylster glowstorm in the waterworks and lyon lorestriker in tower of the helephant both drop every type of mastery amulets. i thought this was like a thing where one of every hundred thousand people got one, but them sylster glowstorm dropped a life mastery amulet. however the odds of this are slim so dont excpect to get one first or even thenth try. i took me a long time. and i've been playing wintertusk for a long time and as far as i know, no one in wintertusk drops a mastery amulet.