MagnusTheGreen wrote:
Moved to Help & Support as the thread belongs here.
As for your question, the simplest way to do this (without Lua) is with layers and events. Make a new layer in the Layers tab, selectable in the top right of the Moondust editor toolbar by clicking on the two item blocks. Let's call our new layer Exit Layer. Place the level exit you wish to use (let's say an SMB3 goal ball) into the level assigned to Exit Layer (this can be picked in the Item Properties tab that appears when you select it). Then, in the Layers tab, hide Exit Layer by unchecking it. This will cause your goal ball to disappear.
Next, make an event in the Events tab, selectable in the top right of the Moondust editor toolbar by clicking on the stopwatch. Let's call the new event Boss Defeat. In the event's settings, under "Layer visibility", add Exit Layer to the "Show" category. This will cause the layer to appear when that event activates.
Now, pick the NPC you want to use as a boss, and, while it is selected, in its Item Properties tab, under "Death", pick your Boss Defeat event, then place the NPC into the level. There is much more you can do with this system as well, but these are the basics.
The "Set as Boss" option (also known as Legacy Boss in the old 1.3 editor) only works for Boom Boom, Birdo, and SMB3 Bowser and causes the former two to drop an SMB3 goal ball and an SMB2 crystal ball respectively when all npcs of their type in the level are killed, and for the latter it causes the game to end and the credits to roll when all npcs of his type in the level as killed. While one of them is on screen, their respective boss themes also play. While this is a useful feature, it isn't implemented for any other NPCs unfortunately.
On another note, I would recommend joining the SMBX discord, as it will allow people to respond more quickly to any help you need, and in real time.