Cloudshifting an existing Fiend hunter
I'm unsure what the judge was talking about, and how this "trick" explained works.

The well-known "trick" takes advantage of the fact that the Fiend Hunter has two triggered abilities. One that triggers when it enters the battlefield, and one that triggers when it leaves the battlefield. 

If you trigger the second ability before the first resolves, the first will still resolve but the exiled card will no longer be linked to the Fiend Hunter because it has left the battlefield (if you used an unsummon effect) or because the original Fiend Hunter permanent no longer exists (if you used a flicker effect, when Fiend Hunter returns to play it is a new object and has no relation to the previous Fiend Hunter which triggered the first ability.)

However, if the Fiend Hunter has been on the battlefield linked to an exiled creature since the start of turn, the second ability's trigger will be placed on the stack when it leaves play. At that point nothing short of a Stifle or similar ability will stop the exiled creature from returning to play.

As Pow-lan mentions in a comment, a possible scenario the judge was referring to is that you use two Cloudshift spells (or other flicker effects), where you "reset" the Fiend Hunter with the first, thus allowing the first ability to trigger again when it returns to play, and then remove it from play with the second before the first ability resolves… but that seems pretty obvious.