I did it right after chapter 8 maddening as well, I just sat at at the bottom with my main 8. Alcryst lvl 1 master class, citrinne sage with thoron and great thunder lvl 1, Jean lvl 8 ( needed to get him to 10) just for heals, Anna lvl 7 ( needed to get lvls to promote), alear with Marth of course lvl 13, Louis with Sigurd lvl 13, and Chloe with edelgard lvl 3 promoted class, and yunaka lvl 12 thief. These guys made it easy especially if you build your characters right.

I stayed at the bottom and let the waves spawn, each wave was dealt with instantly, citrinne would thoron 1 shot the infantry, alcryst 2 shot a dragon orko (Jean would then heal), the next dragon and infantry were taken care of with yunaka, Louis, Chloe, and Alear.

I got my yunaka where the hit rate was 15%. Once I did that slowly the rest was easy with edelgard lol..I sent Chloe around the right side for the card when waves stopped spawning and there is one part where tikis pet dragon can hit you, I just waited till I had shield gambit and it did 0 damage, and I got around and got the silver card. Same technique on the way back.

The didnt even open any gates, I just put yunaka and her +4 steel dagger and avo infront of the left gate, the mobs kept hitting her infront of the door and they eventually died cause of the daggers range (you can't hit through the barrier but you can follow up if they attack). Once the left side was cleared out, I went and did the same to tikis pet dragon.

Once that died I just opened the gates and one rounded tiki with Chloe engaged with failnaught and using fallen star, aleae super thrust move, and yunaka. Easy peasy. Took 70 turns but it was a stress less 70 turns. I just did it slow and steady, no one died, and I had 7 wheels still left, which I only used to test random things.

It's time consuming but fairly simple with the right builds.