The team I used that beat it a couple of times without going 4-0 was Sharena, BunCam, Olivia, and 4* Wrys. This was before I put Gronnblade+ on BunCam though, and she pairs really well with Sharena, so maybe I would have better luck if I tried again.
BunCam can technically kill everything except Celica, but she can't really take hits from anything either because she's got bad resist. She could maybe tank one hit, but any two hits will kill her. Sharena can kill Celica easily, but getting Celica somewhere that Sharena can reach her in a reasonable amount of time without getting one of my other units sniped isn't easy.
Edit: Okay, tried three times in a row and lost by timing out one turn before finishing off the last enemy all three times. The time wasted shuffling around the fort in the middle is so harsh that this feels impossible without an all-fliers team or maybe very specific unit counters that I don't have. I'm not losing units now that Camilla has Gronnblade, but the time is way too unforgiving. Not even sure how I beat it normally now...