Simple: lie and say you've done it before.

As long as you don't obviously mess up, they won't really know. The only actual drawback to never having done is if you're doing in there with really bad gear, cause it's a bit of a dps check. If you know the boss mechanics (easy to find online), then you'll be golden and they'll never know you don't know it. Honestly, it's not a hard dungeon, it just requires people to know the fights, and have a certain minimum of gear.