Get The Song Of Mana And A Cursed Skull.
You Must Survive WIth Lama For 30 Minutes To Unlock The Skull.
Level The Song Of Mana To Max, Then If You Have Been In The Level For More Than 10 Minutes -
The Next Chest You Open Will Evolve A Weapon That Is Eligible.
Eligible Weapons - A Weapon That Has Achieved Max Level, And It's Counterpart (Or Support Item) Is Also Equipped.
If You Have A Banish, I Would Use It On The Skull After You Pick It Up To Prevent Enemies Getting Stronger Later.
The Achievement Pops At The End Of The Run.
Good Luck!