Just beat her after approximately 1000000 tries!
What worked (took a few tries even with this strategy) was bringing in 2 mons with endure (Corsola) and protect (Scolipede) to take 2 decimation hits each. Use revives to keep 'em going to take more hits (fainting after 2 hits each) until Yveltal starts struggling. Then, comes my dude Onix trained to the level cap. Onix has 145 Def. Onix is just enough of a (physical) tank to survive a struggling Yveltal. From there it's just a matter of spamming healing till Yveltal kills himself. Of course, you've gotta have enough revives, hyper potions, whatever to get through it, and Madame X has 1 full restore, so keep that in mind. That's about 8 or so struggle hits.
But basically, tank and then tank some more. I had Nosepass as a reserve, with a bit less Def and than Onix, but I didn't need it. He has Thunderwave though, so he could also be paralyzing tank.
Keep in mind, even with this tanking strategy, your Physical tanks still need enough HP to last. First try, Yveltal struggled Onix to death lol. I gave him a few HP ups, and that 1 or 2 HP makes a difference. Soft resetting also possibly affected Yveltal's attack stat (not sure if it's fixed). I actually think Nosepass wouldn't have helped, as he has slightly less Def and HP than Onix. Any HP boosts still probably wouldn't have been enough.
You could also bring 1 or 2 more mons with endure/protect/whatever to soak up decimation. That's what I had on hand. And btw, the only reason I had enough healing items was b/c I spent time grinding up to the level cap with various mons I tried till I figured this strat out, and figured to add pickups mons (Pachirisu and Ambipom) to the party to make the best of the time.
Also, b/c Jan hates us, this team still needs to be able to beat whathisface before her, as you can't save and swiych teams in between battles.