This achievement can be unlocked on any difficulty. You must simply finish all five levels of Area 4.
If you want to know more about the gameplay, play the Tutorial first. Consider playing Area 4 on Hard, as it is not excessively more difficult, and you will make progress towards
True Completionist in Galaga Legions DX.
See the video attached for a demonstration of Area 4 on Hard Difficulty.
Please either comment below or send me a Personal Message if you have any comments or improvements.