...We know that Jacen got his ass kicked by Mara Jade. I love Mara, but she was little more than a Force-sensitive assassin. Don't get me wrong, that doesn't detract from her at all. She's pretty much a peerless assassin and an experienced/trained Force user. But, hell, it's been stated that Jacen was "second only" to Luke in power and yet she handed him his ass on a silver platter. Thus, once again, "the strongest don't always win". A fight is based more on sheer power.
That said, he likely takes every individual from the New Sith Order in single combat.