Darth Sidious (RotS) vs Knightfall Vader + R2-D2
Scenario: After dispatching Obi-Wan on Mustafar, Vader heads back to his ship to meet with his new master, Darth Sidious. However, Sidious senses deception, and true enough, Vader announces the end of their partnership. Igniting his lightsaber, Vader charges Sidious, who prepares to blast him with a storm of lightning.
R2-D2 is stationed nearby, and alerted to the fight, prepares his oil and blowtorch. He zooms into the frey, beeping, "I'm with you until the end, Master Anakin!"
Who takes the fight? Is Sidious inept, or can he defeat Vader and R2-D2 in the same fight?