You can do the gauntlet without Arthas
I know you might be saying, "what in the world is this noob talking about?" However, it works! After you take the "short cut" you can skip Arthas and start grinding through the gauntlet and head directly for the mount boss.
It's been speculated that this reduces the number of mobs. Only downside to skipping Arthas is that after you'll need to go back and do the gauntlet again.
Lastly, to squeeze out a few more seconds if your really tight you can have your tank pull the mount boss down the hall a ways as he will return to his original position before de-spawns.
Hope this helps!
The Gift of War - a fun-loving guild on WoW's Undermine server