I've been speculating in how to trigger Fade on merc by having him/them stand in fire, but they move about automatically, so i just can't see how that is viable.
Just roaming around in act 1, let some Fallen's hit him for a while, Fade will trigger soon enough.
If you have a strong weapon and good leech, you can just use Treachery as you play normally until Fade triggers, then go to town and switch armor at your leisure. But it is a little tiresome to do this all the time. Fade doesn't last too long for my taste.