First you must do Lost Brother mission with Tarun, then to deal with those bosses the mechanism is the same that dealing with flaming orchid, each attempt to fight them you must complete a requirement that is explained when trying to open their door:
So to access Winter blossom u must kill 1 frost dragon and 2 Energy elementals.
To access Moonlight Aster u must kill 2 liches and 2-3 destroyers
To access flaming orchid u must kill 2 fire elementals and 2-3 Hydra.
once u fulfill their requirements u can access their door, this must be done each time u want to access them, also each one of those mobs must be killed inside asura palace, they are caged on rooms all through the palace:
The Black, Gray and Blue Circles have: Frost dragons/Liches (is a crack in the floor with mobs in separated rooms), Hydras (in a hole) and Massive Energy Elementals (is a room that must be accessed through a crack in upper floor) respectively.
The Orange Circle have Destroyers (accessed through a crack on upper floor)
The Teal Circle have Massive Fire elementals (accessed through a crack on upper floor)
Winter blossom is inside the Black circle Spot, in a separated room while flaming orchid and Moonlight aster are in separated rooms in the roof of the building.
About their loot, the first time they are killed would drop 100% their respective feather, after that they wouldn't drop it 100% or probably wouldn't drop it anymore, then their average loot is at least bad, from 15 runs my only notorious loot was:
x1 glacier robe
x1 White Gem
x1 Oriental shoes.
it is still unknown if they drop something worth with very low rate but seems like those bosses aren't worth to do in a daily basis beside to unlock bosstiary.