Here is the full list of all 35 Millennium 5 - there are The Battle of the Millennium achievements.
Win more than 85 fights during the Showdown
Defeat Merryll with at least five characters
Defeat 40 Duos of Butcherosaurus
Defeat Merryll with Jezabel
Defeat Lord Jack with Jack
Visit the Shower Room of the Arena
Leave Ancient Myst for Mystrock
Finish the Pit of Infinity for the First Time
1000+ fights
Make The Bear mad at Salome
Kill all animal kings
All secret rooms found
Kill Dumberork
Find the Shop of the Gods
Collect 10 Flowers in Jungle Kawa Kawa
Cuz Nothing Else Matters
Kill Glurose
Kill Hippoquake
20+ secret rooms found
Visit the Top of the Abandoned Lighthouse
Complete 6+ quests
Kill Tukulu Nation
Talk to the Healer in Cactus Crest
Kill Monkey-See-Monkey-Do
Held more than 99,000 Gold
150+ items found
Discover the beach
Have Blondie join the party
See blond girl for the 1st time
Kill 100 Titan Beetles or Mother Crabs
Find 1 secret room
Spend 40+ hours on the game
Play 1+ hour
Complete 1 quest
Make The Bear mad at Marine