I saw a guide down there from SladeKilla, but I think I have a somehow better guide.
Control the use of your MP to save it for Heal. Just beware when Shadow's HP is lower than 300, and when Nights HP is lower than 700. Start the battle with an Energy Beam from Shadow and a Star Rain from Nights. Keep at basics attacks after, always watching ur HP. When u see an opening for Star Rain, try using it, as long as it saves MP for 3 - 5 more Heals (denpending on Reala's HP). Use Judgement Night and then SP outbreak later on.
As for the Beast, just Attack till Sonic's HP falls too low, and then use the Super Form, use all the Extras and as many attacks as you can. After ur super form ends, use Heal and then 2x - 3x Spindashs and he'll be done for.