The phrase "march up to cannon mouth" is an idiom that refers to facing a daunting and dangerous situation head-on, as if marching towards a loaded cannon. Synonyms for this expression may include "brave the lion's den," "confront the firing squad," "face the music," "step into the lion's den," or "walk into the belly of the beast." Each of these expressions denotes a similar sense of fortitude and courage in the face of adversity. Whether facing a difficult boss or overcoming a major obstacle, these expressions can be used to showcase our determination and bravery in challenging situations.