March up cannon's mouth is a phrase that refers to the act of bravely approaching an intimidating or difficult situation. There are a number of synonyms that could be used in place of this phrase, such as "face the music", "step up to the plate", "meet the challenge", "confront the problem", or "take on the enemy". Each of these phrases conveys a sense of courage and determination in the face of adversity, and can be used to encourage someone to push through their fears and take action in difficult situations. Ultimately, whatever words we use, the most important thing is to embody the spirit of bravery and resilience that enables us to overcome the obstacles that life throws our way.
