Yveltal is more a direct win than Xerneas because Yveltal, canonically, can cause a self-feeding Anihilation Event on all life when it itself dies. It's even brought up on the latest dex entry as game-canon confirmation.

So imagine it is fighting and somehow loses, gets crushed by a billion lions - the lions will all then immediately start fading away into a sphere of doom that expands to consume all of then, and then retracts back down to generate an egg. It's a tie. At least, it's one until a short while later when Yveltal births again.

Yes, the ultimate weapon in Gen 6 is basically just a way of using both Xerneas' and Yveltal's powers as one. But for the purposes of beating a billion lions, really, Yveltal is the important component. That was why I only put it up.

Ghost types

Hadn't even considered that. Good call. Ghosts on the whole could also easily be stall wins.