To stop people staying in the corner, or elsewhere for that matter, have some special ghost that will spawn where the player is every few seconds. Give them a warning by flashing the ghost after spawning there or something. And instead of this ghost following the player, maybe have it fire off a burst of ectoplasm.
As for the game, it is a little boring tbh. Maybe too many ghosts from the get go? Not sure how you're doing the bullets - I assume they're using the bullet behaviour - but it might be better using the turret behaviour, then you can use the predictive aiming. When I was playing it, when the bullet missed a ghost it aimed for, I was kinda pissed 'cause it was out of my control.
Also, you could add a powerup that appears every so often that makes the player invincible but is also able to smash through the ghosts and kill them upon contact.