IIRC, Soul Dew has no effect in Battle Tower.
I think you should add Trick to your Starmie for sure. It makes a lot of stuff set-up fodder and nets wins. With BoltBeam + Surf, Grass Knot isn't too necessary. I'd consider Roost for the last slot in Scizor's moveset if you go with Trick. Then use Life Orb. This is crucial as you can Substitute on stuff like Ghost/Dark/Grass moves aimed at Starmie, then Swords Dance up. At +6, with Life Orb, Scizor is able to defeat even stuff like Charizard in one blow, something it can't do without Life Orb.
If you choose to use Latios, I'd consider Focus Sash or Yache Berry on Garchomp so you have more insurance against Ice specialized trainers. Focus Sash protects you from Critical hit Ice moves whereas Yache Berry doesn't. Sash also helps against OHKO move users. But Yache can leave you with more health to sweep with.