This video covers various exploits that I've found today in Hellfire Citadel; I will post instructions for them below:
Exploit 1: Flying from Mannoroth to Upper Citadel
Step 1: Have a warlock set up a summoning portal at Mannoroth and portal back down to lower.
Step 2: Mount up in a friends 2-seater mount and click the Upper Citadel portal
Step 3: Wait for the Arcane buff to wear off (takes a couple of minutes) and then have the lock summon you while in the 2-seater mount.
Step 4: Accept the summon and you'll fly through the world from Mannoroth to Upper Citadel, pulling mobs and glitching through the world on the way.
Exploit 2: Flying in Upper Citadel / Mannoroth (Allows you to skip any trash / most bosses)
Step 1: Mount up in a friends 2-seater mount and spam click either the Upper Citadel / Mannoroth portal (you will be able to fly in the opposite of whatever you pick)
Step 2: Have you friend run to the other portal, click it and get off your parters mount, you will then be ported up to the appropriate location and be able to fly for up to 60 seconds.
Exploit 3: Destroy Mannoroth's Spires
Step 1: Follow Exploit 2 but take the Mannoroth portal as your first portal
Step 2: Once you've been ported up to Mannoroth fly around and click the spires individually, this will cause them too explode.
Video attached below: