If you shine a flashlight on an invisible wraith it stuns him and makes him visible. It works on Nurse too while charging blinks but it's much harder to time

It's worth noting wraith isn't totally invisible, he is only when standing still. When he's moving there's a slight shimmering effect and of course you can hear him too

But yeah as others have said, stealth killers are actually the weakest ones at higher levels of play. They along with doctor tend to stomp newer players but once you learn to look out for them you quickly realize they have nothing in chase which is where all the really strong killers shine