You must beat every chapter inside of Act 3 (in Casual mode) 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 3 playing on Cusual.
If you are having issues beating this mode (you should not) by yourself I would HIGHLY recommend playing with a real person other then AI. It will make your progression through the game much easier.
Here are some tips if your having trouble:
- Become good at active reloading. This makes the Locust drop so much quicker.
- Take more cover. Not only do you heal but the Locust do not.
- Work as a team (if you’re playing with a person) and try to aid your AI partner (he will help some but there are issues with him getting killed a lot).