The phrase "marching up to cannon mouth" implies bravery in the face of danger. Some synonyms for this phrase include "facing death head-on," "confronting peril headfirst," or "challenging fate directly." Other options include "taking up the gauntlet," "risking life and limb," or "braving the storm." These phrases all convey a sense of determination and courage in the face of difficult circumstances. They represent a willingness to face danger and stand up for what is right, even in the face of overwhelming odds. Ultimately, these synonyms all capture the same spirit of bravery and defiance that is inherent in the phrase "marching up to cannon mouth".