What's the most efficient way to SR for Calm Thundurus? I already know it's max possible stats for Calm Nature is: 154 / x / 90 / 145 / 110 / 131

So far what I've been doing:

- I have the first pokemon, Gardevoir, with a Calm nature and Ability Synchronize, fainted.

- I lead off with Smeargle, with a speed stat of 130. This way I see if Thundurus' speed is 131.

- Use Spore. If Thundurus' goes first, it has a speed stat of 131 = 31 IV in Speed

- Switch into Staraptor, with a speed stat of 132 and HP Stat of 153, to use Final Gambit

Here's the problem, Thundurus' has a chance to use Agility before I spore it, allowing it to outspeed Staraptor and possibly wake up before Final Gambit.

Is this the most efficient way possible? Or is there something else more effective?