Since game ver. 3.5, there is only 5 masters.
Since game ver. 3.16, how to achieve this has changed
it now required to kill the following boss:
(This can be done in permanent league, as achievements are not specific to temporary league (league =season of other games))
Also a tip: when you buy kill, you can always use skill that is linked to Culling Strike (or similar passive skill) to land the kill blow
Alva: The Vaal Omnitect (boss of the Temple of Atzoatl)
difficulty: 1/5. Easy to spawn by just linking the room by opening doors in the temporal incursion to the past; survive and kill the boss in the present day Temple of Atzoatl
Einhar: Farrul, First of the Plains (the cat boss)
difficulty: 3/5. Easy to spawn by buying the portal beast, kill the beast to open portals, and enter the boss fight proper. Can hire someone else to kill the boss in the party
Jun: Catarina (the Syndicate Mastermind)
difficulty: 3/5. Yet another easy to spawn boss. Just keep on acquire Syndicate intel by killing Syndicate wing leader in their Safehouse. The boss is yet another dps check. Manage the self-explode skeletons that follow you very slowly, or manage your pace on the totem rotational AOE are more important. Yet another boss that you can just buy kill
Zana (Kirac): Shaper and Elder
difficulty: 4/5. The patch note is not that clear but it seems imply the Uber Elder kill instead of killing Shaper and Elder in separate boss fight. The boss is one of the pinnacle boss of the game, but it is yet another easy boss if you have enough dps and enough defensive layer to tank or evade or block or migrate the boss' dps. Just buy kill if you can't beat it, a lot of people are willing to offer boss kill service in party play
Niko: Aul (boss of Primeval City biome in the Azurite Mine)
difficulty: 5/5. Delve is one of the game mechanic of the game as an alt end game. The boss is very very rare that you can only found him (it) in delving into the deep mine. The monster in mine however scale with the depths, so that when you find him (it), probably he (it) will be very tanky due to the scaling, plus a few of nasty randomly generated area mod that cannot be rerolled once generated and displayed in your mine chart. Even the dev claimed that the spawn chance will be buffed in October 2021 patch (3.16), the boss probably still very rare. And you cannot buy the entry to this boss, you need to dig the mine yourself. You can buy kill, but in solo, you have one life (instead of 6 portals = 6 life aka 5 respawns), so that i am not sure the party member was killed, he can rejoin you or not (in case you survived)