Easier if you watch my video but heres a text guide
After an unskippable cutscene open either free chests in front of you, ones slightly off set to the left and one slightly off set to the right, open both as they are free and give you lots of koins and hearts although you only need one to open the door. Head straight forward once the doors in front of you is open and don't veer off as you will come so shao Khan's hammer which you can pick up by pressing A, do that, and then immediately behind where you picked up the mallet is a large door that has a brokenish looking section, swing your hammer at that section to break open a big enough gap to pass through by pressing X. Once in the next area and head left by hugging the left wall, you should come to a section of wall which juts out slightly but has a section which is broken but formed, press X to swing you hammer at that section to break open a hole that will allow you to pass through. Once through walk slightly forward heading left and you should see the gong quite visible, swing your hammer at the gong by pressing X and voila, achievements yours.
I've tried my best to explain this with text but it really is easier to watch the video I've made below