You defeated a major opponent using only your default equipment.

This is actually quite a hard task. The major opponents are Rajani, The Old Dapper Gent, Jeyne and the Reborn God. You must have unequipped all items to your default equipment, and make sure you are not wearing any rings or amulets as these will void the achievement.

This is easier done with a friend, as they can still use all of their normal equipment and defeat the boss as you just hide away.