Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General NPCs in GM Mode Levels

Depends on what you need to do...
GM mode will give you the basics in level preparation.

If you want to do more intricate stuff its probably best to lay the level and main characters out in the editor and then just use the GM mode for "Random Encounters" and the on the fly stuff that you drop in mid game.