How do I teach Doublecast to a Summon?

[Tavrobel]

Well acually I'm stuck at the Sin encounter, oh how I hate Giga-Gravition
And I can't think of anything else than that

The Arena can be found in the Calm Lands right?

Yes. Don't use Aeons during the fight with Overdrive Sin. It causes the Overdrive bar to fill up many times faster. Use Auron, Wakka, and Tidus, or Kimahri for most of the fight. If Lulu has Doublecast/Flare, use that. Don't use Holy, since the cooldown time is too long. Quick Hit is a wonderfully useful option for either Tidus or Kimahri. Hastega everyone, and attack.

Alternatively, you can have Rikku mix up a few Trio of 9999 or Quartets of 9, and attack like that.

You should be able to get 5 Three Stars by playin Blitz also....just resetting until it is offered as a prize.

I had begun with the assumption he didn't play Blitz.