You can get under the dueling area by using the Hog currently in beta.
Start the duel above the ground.
Mount up, go to where I say in the image, dismount. Yes you will fall, so make sure you have at least half health.
Images as followed:
Image 1: This is where you can get under
Image 2: This is where you go.
Image 3: This is how you mount up.
Image 4-5: This is how you fall.
Image 6-7: This is in the under duel area.
Image 8: This is teh L33t Arena NPC's that don't hurt you, and you can't hurt them
That is all, just wanted you guys to have fun dueling in a place where you typically can't.
PS. It is also possible to get under the floor of the under area... just have someone fear/poly you while you stand on the boxes in there. You can get on the boxes by going to a glitched place next to the boxes, and mount up on ur flying mount for a few seconds.