How do I get to the ghost in 2-6?

I want to know as well... i can reach that place but the ghost never talks neither attack. Just keeps following you from left to right... if you try to exit the place he wont follow you either... i think its just for show.

There's the nazguh ring it makes them vulnerable and follow you then you can get the ghost buster achievement.

Wear the nazguh ring to force it into a solid form and stand under it. Abandon your fear, science will protect you. Thunder will hit the ghost instead of you because now it is corporeal, and will act as a lighting rod, after a few hits it will die.

You can also attack them and then turn around while the weapon is heading towards the ghost. That way they will become tangible and take damage without you having to buy an item