This achievement is stackable so it will unlock if you play at a higher difficulty level.
You must beat every chapter inside of Act 5 to unlock this achievement. You are allowed to play the chapters in any mode: co-op, single player, or a mixture of both. The achievement will unlock as long as you have credit for beating each chapter in act 5.
If you are having issues beating this mode by yourself I would HIGHLY recommend playing with a real person other then AI. It will make your progression through the game much easier however this is not a requirement. It can be done by yourself.