Divinity: Original Sin 2 - Definitive Edition

It doesn't matter what difficulty you played the entire game on, it only matters what difficulty the game is set to when you talk to your ship in the final scene of the game. Beating the game on this difficulty unlocks the achievements for the lower two difficulties as well.

I played the entire game on Explorer difficulty and switched it to Classic mode at the end. Unfortunately classic is the highest it can be set to, and you can only play on Tactician or Honour mode by starting the game with that mode.

Linking the other two difficulty achievements that can be earned with this method.