Among the multiple arena teams I run, one of them is a Nihilus lead.
After much experimentation I think I have one down that is effective against even Zaul teams.
Nihilus, Vader, Sith Trooper, Sith Assassin, Dooku
Even with Zaul's bonus turn meter at the beginning, my Sith Assassin will go before Zaul, but usually not before EP. So, some of this is based on the luck that SA doesn't get stunned at the beginning. I always have SA use her Dark Shroud to give the rest of the squad some nice turn meter, which means, I can unleash Nihilus and Vader special before anyone gets a chance to throw tenacity up, or an enemy Nihilus to add to my cooldowns.
From there, it's just a matter of whittling the opponent down.
Biggest advantage of a Nihilus team is that you can't critically hit anyone, so GK's taunt never pops, and Zarriss's Special never activates
Sith Trooper is really hard to kill in this squad, especially once he gets his counter ability up.
I sometimes flip out Vader for Sid, or sometimes Savage (maybe Savage more once I zeta him). I rarely use EP, as his basic can still critically hit on the non-targeted enemies (Jedi and Rebels)
Overall, I like this squad. It's usually a 3 minute plus fight, but once they enemies start dropping, it goes fast.