itch.io

The best way is by designing a level that requires the shift to absolutely move forward. Like put the shadow mob on the ridge that is in jump height on the edge. That way they have to kill it first before they jump on the ledge without dying. You may also put some text in the world at the first monster letting them know that something in the shadow world seems to be blocking they way. This way you force the users to have to engage with that system and you give them some affordance by indicating that their is something there. That would be one way at least.

You may want to grow a reveal from the character to the edge of the screen. That or put some sort of poof animation like ninja smoke on the character so that it is noticeable that an action was performed. 

Keep it up is a very nice game.