You will get this achievement by playing through the story. After you give Supplies to the church, you'll bring the truck back to the Slums and that will end Chapter 8. Two more quests and you will get a quest called "Devastator" Where they talk about doing a certain task. Once you complete this you will have Completed Act 2.
Quest *Spoiler*
*** Spoiler - click to reveal ***You will go to find a bunker with Jin, you will progress to blow the door open and this will leave a nice fast travel to the Jungle.
Short Version:
Chapter: 9
Quest: Devastator
Hope this Helps! :D