Strongboxes Spawn Randomly On Maps And Levels. Usually It Is Tagged With A "Guarded By 3 Packs Of Monsters".
- Using An Orb Of Alteration, Chance, Or Transmutation On It Is The Easiest Way To Get This Done.
You Can Also Use Most Other Orbs To Accomplish This. (Regal, Chaos, Exalted)
- Hover Over An Orb In Your Inventory To See What It Can Be Applied To.
Upgrading Or Changing The Strongbox In Any Way, Before You Open It, Nets You This Achievement.
Good Luck!