Do not forget Luke does know the lightsaber form Vaapad. Close enough the Boba, armor destroyed and a lightsaber through the chest. Also Bane's orbalisks would suffer the same fate as Boba's armor. And the outcome would be the same. But it would be a massive Force battle.
Leia's no pushover. Although no match for Bane, she could handle Zannah. She's of the Skywalker line and is no chump. Han would be pouring blaster fire at Fett and Fett would be pinned down.
Caedus stated that Luke is the greatest swordsman and Jedi Master and Bane would be hard pressed to defeat Luke. I never said Leia could defeat Zannah, but could hold her off long enough for Luke to kill Bane. Luke engages Zannah and the Sith girl doesn't stand a chance. Just as a thought, IMO Vestara is stronger than Zannah. My apologies on Vaapad - I did mean shatterpoint.
Han and Fett would be engaged in a nasty firefight. After Luke finishes off Zannah, he aides Han and kills Fett. Battle over.