POE 2 Boss atlas skill tree

First book:

Take a T15 map and get 6 modifiers on it (i.e. use exalts until you can put any more modifiers on it).

Use map on an atlas node that has a "boss" along with "irradiated" or "corrupted" in order to get +1 levels.

Second book:

Make a T15 map like above and use a Vaal orb on it. If it doesn't turn into a T16, do it again with a new T15 map. Repeat until you get T16 map.

Use map on an atlas node that has a "boss" along with "irradiated" or "corrupted" in order to get +1 levels.

Remaining books:

Same strategy but the atlas node has to meet the special criteria -- I haven't dabbled into these books yet so I can't give any insight here.