In my mind, the boss-fight will be in several stages, with Ciaran entering after Sidious has survived the first few, and then she manages to put the weakened Sith Lord down after her dramatic reveal.
In particular, I imagine that Sidious will first be confronted in his office by the Jedi, while Ciaran's various heavy hitters surround the place to either lend aid or to slow him down if he tries to escape. Ciaran, Thrawn and a few others will act as overwatch, probably on a spaceship over the city. Perhaps with a small fleet to stop him if he manages to get on a ship of his own.
Sidious, either upon noticing the signs, or after being attacked, decides that the current fight is not to his advantage and tries to disengage and flee. The first epic battles and assorted chase scenes follow, all the while Sidious tries to enact his various emergency plans (Order 66, hidden battle droid groups, bombs to take the Senate hostage, whatever) He might be killed and enter one of his clones, if I remember that discussion right.
Of course, all that is not enough to stop Sidious, who manages to escape the encirclement and hide long enough to home in on a secret space-ship with which he might escape the planet. Ciaran, of course, notices this and tries to intervene. Her own ship comes close, and to the surprise of everyone, she jumps from the ship. Turns out, that she also learned that one force style that makes you super tough and strong people have been mentioning. The reason for her jump was that, upon seeing Sidious' ship she noticed the perfect opportunity to combine her strenghened body with shatterpoint.
Sidious has just managed to loose his pursuers and is taking off. Ciaran, imitating a shonen protagonist, impacts on the ship feet first, manages to somehow break something critical thanks to the magic of shatterpoint and rides the falling ship down right into the ground.
After that, there is the final final battle between Ciaran and Sidious, all the dramatic reveals à la "Of course I knew that you were Darth Sidious all along! Did you think you could hide from the one and only Dart Traya!?" The fight finally (and dramatically) ends when Sidious' body is destroyed (
again, if he has possessed clones before) and Ciaran uses shatterpoint to destroy his force shadow / force phantom.
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I think that got away from me for a moment there. Sorry for any lack of coherence, it was pretty much my stream of consciousness while thinking about how Ciaran would come to have a final battle with Sidious as mandated by
@Dr. Snark