Coordinate with your partner if you can. With that few points, each of you is going to be able to pick one or two things to do.
Correct me if I'm wrong here as I don't play Ynnari (yet), but won't an opponent just not shoot the throwaway units and not trigger SfD?
In any case, you're gonna have a hard time triggering SfD on Reapers. I'd just rely on Yvraine to do that with psychic powers on one big unit. Take something that's a little more "in your face" for the natural SfD triggers.
Also, if you're worried about Alpha Strike, try a Wave Serpent for the Reapers.
You can get a Ynnari Battalion for 800. Example:
Warlock (pocket conceal for Reapers)
Dire Avengers, 2x6 + Exarch, 1x5 + Exarch
Dark Reapers, 1x8 + Exarch, all Reaper Launchers
Wave Serpent, Shuriken Cannons
If Battlescribe is right, that's 796 points. With just the core of Yvraine, Warlock, Dark Reapers, and Wave Serpent, you have 256 points ish to make 3 troops choices from.
Yo ho, yo ho, a drug-fueled BDSM space-elf pirate's life for me!
Can I get a Roll Tide?