Here is the full list of all 12 Nemesis Realms achievements.
Play one battle in any stage
Play 15 matches
Win 50 matches while playing as the boss
Win 50 matches while playing as the hero
Play 30 1v1 matches without customization
Win 300 matches as the boss
Win match in lair using only your head
Successfully perform footstool to one of the dragon heads
Hide under water for short amount of time
Land 5 successful dives to boss as the hero during a match
Play a custom match where all characters have min health
Play a custom match in Luna where the hero can't jump