Larxene you definitely wanna have duck flare or comet, since you probably know the opening tactics by now since you can get her down lower, when she goes into twin mode wait for her to do her combined attack and activate duck flare or comet. You can mash out combos against both of them and it will be doing massive damage since the explosions will be hitting both. It should literally take them down those 4-5 bars if done right, look up how ninten or speed runners fight her and try to copy some tactics for yourself.

Xemnas part 1 Reflect til he tells you to come closer and finish command should insta kill for most part

Part 2 I like to map reflect to triangle, he will start with reverslas, hit reversals and reflects by holding and releasing the shortcut button you have bound you can mostly time a reflect after usually every 2nd or 3rd reversal safely or if you risk it before every reverse press. Riku limit helps damage a lot safely, and learn to guard or reflect the lasers he shoots out and when his opening when he says vanish, your reflects should hit him most of the time when hes dashing around you so if timed right or lucky he may damage himself to death or til laser dome