From Halopedia, the Halo wiki
Ramblin' Man is an Achievement in Halo Wars that is unlocked when the player trains over 100 Marine squads from an Elephant on Anders' Signal.
It is worth 5 Gamerscore.
Activate the Pain Train, Bountiful Harvest and Sickness Skulls and wait for a large amount of resources.
Constantly purchase Marine units out of an Elephant and send them to get killed, repeat the process until the achievement is unlocked.
- The achievement is a reference to the song Ramblin' Man made by The Allman Brothers Band.