Bestiary Boss

So I didn't play in the bestiary league so I am completely lost on how to do the "Defeat a Bestiary Boss" challenge. I bought a unique bestiary thing on poatrade and fought it with no such luck on the challenge completion. I saw that in youtube videos bestiary bosses have portals to a special arena. How do I know which beasts give this achievement and which do not, and how can I search for them?

Go to the menagerie and search for "portal", win the fight and portals will open up to the boss area.

Kill him and then you get to craft the bosses special thing onto your gear.
"Aspect of (insert boss)". Its like exalting gear, so it wont take away a crafting slot, but you´ll still need an open pre-/suffix to do it

Those special beasts are very rare and some have map tier requirements (see wiki). You can use scarabs that spawn multiple red bests in maps but you migh as well end up buying one of the beasts