You can find the requirements for this achievement in the game:
Menu -> Journal -> Achievements -> Character , companions sub category
After obtaining a companion you must summon them and then complete all 4 below tasks.
1. Defeat a world boss with any companion.
2. Kill a group dungeon (4 man you can queue for) boss with any companion
3. Kill a delve boss with any companion
4. Complete a dark anchor with any companion
Mirri is located at Doomvault Vulpinaz in the Northwestern portion of Blackwood.
Bastian is located at Deepscorn Hollow on the Southwestern island in lower Blackwood.
Ember is located at Draioch just north of Gonfalon Bay in the South of High Isle.
Isobel is located at Castle Navire in the Southwest coast of High Isle.
Sharp-As-Night is located righ tby the city gates of Necrom in the Telvanni Peninsula.
Azandar can be found around the outskirts of Cipher's Midden in Apocrypha around the middle section.