If you're after completing this achievement ASAP, open the spoiler. If you are after enjoying the story without spoiler, just continue playing as you would and this will eventually unlock.
Things might be a little different for you as choices affect how the story goes. *** Spoiler - click to reveal ***First, try to remove the prismarine glove from your hand. Then you'll need to jump by pressing , tapping it afterwards before pressing . Cover up the mouth and afterwards spot the hole in the ceiling. Leave a sign and finish the conversation with Petra, before going to Champion city. Walk away from the screen to confront Stella. Another conversation with quite some choices to make.
After that, either choose to sneak or to fight your way in. (Sneaking is easier)
(if you chose to sneak your way in, throw bones twice into the puppies' places. Confront Stella and Petra will get her sword back.)
((If you chose to fight your way in, you will fight 2 body guards with a short QTE, press 3 the 3 buttons and then and then in the 2nd fight press the 3 buttons. Then simply disarm Stella and Petra will get her sword back. )
On your way back, use to avoid the explosion, then twice, and then cover the hole again. Upon reaching Beacontown again your achievement will unlock.