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