Diablo III

In order to complete all of the quests in an Act, you must go through the entirety of the main questline, which involves questgivers like Leah, Tyrael, Deckard, etc. You can skip/miss these quests if you join a friend who is a few quests ahead of you in a co-op game, however you can load up the quest that you were on once you're done playing with your friend, from the main menu of the game via "Quest Select", so the achievement is not missable.

The Act II quests are:
The Siege of Bastion's Keep
Turning the Tide
The Breached Keep
Tremors in the Stone
Machines of War
Siegebreaker
Heart of Sin