Sorry, I was on a road trip.
The Shadow King is a psychic parasite, which is both a strength & a weakness. It's a strength because he can hop from host to host, but a weakness because without a host he can be taken down by a telepah of reasonable power.
(Professor X defeated him when he was fairly young, in a classic tale which also included a young African pickpocket with white hair).
Emma's power has fluctuated, but I recall one story that she makes up for a lack of Phoenix-level power by her finesse & skill. But that was a story that had Rachel Grey jobbing to her, so it might not count for much.
As for the Shadow King & T'Challa... he got into Wakanda by possessing Gentle. He found his way into T'Challa because BP was distracted while battling mind-controlled Wakandan guards.
In the story, that was his method. Not direct psychic confrontation, but more like a virus that corrupted the hosts slowly while their guard was down. You can't use psychic defenses against a foe unless you know the foe is there.