To unlock 'My Empire of Dirt', you must complete the game on Soldier difficulty. This is the second out of four difficulty modes available, and can be done on your first playthrough.
There are eight chapters within the game, averaging at about 30minutes in length.
Soldier difficulty isn't tough, so you should manage without much help. There are a few frustrating parts where you may get overwhelmed, but just persevere, and remember, dying will restart you at a checkpoint with full HP, even if you didn't pass that checkpoint with full HP.
This achievement stacks, so playing through on the harder Centurion difficulty will also unlock this.