Questions about the Heroes 4 map editor.

but you can't trigger a battle using a trigger... or can you?

Yes, you can, I don't remember the specifics, and I don't have the game to test it – but I'll tell you from memory:

In the Army property -> Events -> then there should be a category selection – choose trigger events -> new (name) -> choose battle, then this army -> enemies -> and what will happen after victory/defeat

Then, when the time comes, just place it in the appropriate location (City, army detection, local, calendar event, etc.) -> name of the event.