Martial Marsh Master is an achievement in Plants vs. Zombies 2. It is achieved by "unleashing" the Gargantuar, but more specifically completing Jurassic Marsh - Day 16. It is worth 20 points on Game Center and 2,000 XP on Google Play Games for Android devices.
Its name is a portmanteau of "martial arts master," meaning a person who is a professional or is very good at a martial art, like Kung-Fu, "marsh," referring to the period and place Jurassic Marsh is set in, and "mash", referring to the Gargantuar.