It's hard to get Maul to go first because he is so slow. And all those Sith are big on AOE which invites mass counter-attack from Phoenix. So you have to boost your potency to land those controlling debuffs like Stun and Daze. That means Palpatine lead. Also, Rebels and Jedi take passive damage from Palp's leader ability, so using him vs. Phoenix is really the most effective lead.
Vader gets huge TM boost vs Phoenix, so he goes first with Force Crush. Palp lead ability gives Vader a ton of bonus TM, so he goes again with Culling Blade. Then Palp should go next with his AOE; if his potency is up enough, he should get a stun or two (or more!), and might even get another bonus turn. Now you can AOE with Sion and Nihilus ... Sion should go first to protect Nihilus. If Sion lands some Pain, he might get another turn too.
When Maul finally gets around to moving, you can lay down the Daze which will shut down Phoenix if they are not already. Maul is very strong against Jedi, so try to use him to attack Ezra and Kanan.