Here is the full list of all 26 Has-Been Heroes achievements.
Fulfill the Grand Soul Orb 100 times
Unlock the Black Knight, Thanatos
Play through the game seven times
Play through the game six times
Play through the game five times
Fulfill the Grand Soul Orb 50 times
Play through the game four times
Play through the game three times
Fulfill the Grand Soul Orb 25 times
Play through the game once
Play through the game twice
Fulfill the Grand Soul Orb 15 times
Fulfill the Grand Soul Orb ten times
Complete the Prologue
Fulfill the Grand Soul Orb!
Fulfill the Grand Soul Orb five times
Unlock all skins for Metacles the Monk!
Unlock all skins for Favian the Bard!
Unlock all skins for Kedi the Tiger!
Unlock all skins for Khar the Dwarf!
Unlock all skins for Crux the Warrior!
Unlock all skins for the Wrestler, Florencio de la Torre, aka "El Antropólogo"!
Unlock all skins for Zakwas the Bodyguard!
Unlock all skins for Roho the Robot!
Unlock the Magician, Edran
Unlock everything in the game!