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