Essentially you want speed run the exploration areas on story difficulty until you come up against several waves of corrupted machines, these give the best XP and more seem to spawn.
It can take a while to find these areas.

This is one that I found.



As you can see I am level 15 in the screenshot.

What you want to do at this point is set the game to hero difficulty, clear the enemies and then take environment damage as per below. You can switch between the difficulties if it's too challenging.



Once the character dies the game will reload you to the start of the area, you can just keep repeating this, you won't bank any studs but your XP will remain.



This was my last character and massively reduced the time to level.
Hope this helps :)