This is written assuming you are just cleaning up the "win as" achievements and don't care about the other achievements like win type, boss mob kills, or NPC recruitment. Using the below setup I was able to win in 32 turns, or about 10-15 minutes.
Start a new game as the Kingdom of Gilden with the following game setup options. Any settings not mentioned can be whatever you want.
Map: Skarps' Veil (tiny)
World Size: Tiny
Wildland Frequency: None
Production Pace: Very Fast
World Difficulty: Novice
Monster Frequency: Moderate
Pacing: Fast
Number of Opponents: 1
KvE Balance: Even
Typical Intelligence: Novice
Surrender Threshold: Often
Victory Conditions: Turn off all but Conquest.
Found your city on the spot with the best production. Open the unit designer and edit the Militia unit and add the traits Brute, Enimity, and Veteran. Set your city to build two units. Circle around your city poking at mobs and treasure until the units are ready, and then follow the road to the other side of the map until you find the enemy city. Chances are at this point it is empty, grab it and find the soverign. Whack em with your army (or laugh as they run straight into a large mob like they did for me twice).