So that new SpotPass paralogue just got released. It has you fighting mirror images of your army, stats, skills, and all. This is where being over-leveled will come back and bite you in the ass.
it's mind-crushingly difficult D:
EDIT:
Huzzah! I may have found a winning strategy for this.
1) Cheat the weakness system a bit. I reclassed Chrom to a paladin for the sake of the mirror Chrom having the weaknesses that all mounted units have. I then outfitted MY Chrom with the Exalted Falchion, 2 Beast Killers (forged to maximize their hit rate), and 2 elixers.
2) Have your avatar able to wield magic in one way or another. Have him/her wield either/or Celica's Gale or Waste (if for some reason you're set up as a sorcerer). Again, max out the hit rate and raise the strength as much as you can on those.
3) Have Chrom and Avatar take on the opposing army head-on by themselves, healing with elixers as needed.
4) Fill out the rest of your available space with weak, expendable SpotPass units.
5) Have one or more of those said units be healers. You're gonna need them to heal Aversa. If you want to recruit her, she has to kill the mirror image of herself.
Also the first of the Series 2 DLC is out now. It's part of what's being called the Challenge Pack which is also supposed to be mind-crushingly difficult.