Full list of all 20 Boss Rush: Mythology achievements worth 1,000 gamerscore. It takes around 0-0.5 hours to unlock all of the achievements on Xbox One.
Defeat the Minotaur
Defeat the Hut on chicken legs
Defeat the Garm
Defeat the Valkyrie
Defeat the Gullveig
Defeat the Gryphon
Defeat the Hephaestus
Defeat the Skadi
Defeat the Anhur
Defeat the Three Bogatyrs
Defeat the Neith
Defeat the Ares
Defeat the Ammit
Defeat the Svyatogor
Defeat the Loki
Defeat the Thor
Defeat the Anubis
Defeat the Chernobog
Defeat the Ra
Defeat the Zeus