Here is the full list of all 20 Supreme Ruler The Great War achievements.
Win a Sandbox Game with the Victory Condition set to Approval Score
Win a Complete Victory Game from any nation at any difficulty level
Win a Game with the Victory Condition set to Hold Objective
Win a Sandbox Game with the Victory Condition set to Economic Score
Win the Russian Campaign Game
Win a Unification Victory Game from any nation at any difficulty level
Win the First World War United Kingdom Campaign Game
Win the First World War German Campaign Game
Win a Sandbox Game with the Victory Condition set to Military Score
Win the First World War French Campaign Game
Any Victory as any nation in the 1914 Sandbox
Win a Game with the Victory Condition set to Sphere
Win a Game with Victory Condition set to Capture Objective
Win a Sandbox Game with the Victory Condition set to Diplomatic Score
Win a Game with the Total Score Victory Condition from any nation at any difficulty level
Win a Game with the Victory Condition set to Tech Race
Win a Capture Victory Game from any nation at any difficulty level
Win a Sandbox Game with the Victory Condition set to Technology Score
Win a Sandbox Game with the Victory Condition set to Victory Points
Win a Capital Victory Game from any nation at any difficulty level