My power level is 155k and while I haven't managed to 3 medal it, I've gotten her upgraded. Here's what I did:
Make 2 strong teams. Then sort by speed and make 2 crappy fast as hell teams. Make sure your strong teams are 1 and 2 so they can kill the mobs at the start. Pick a strong ally with decent speed and place them all in the top most starting area. Ideally, you're whole team will go to the right. What usually happens though is 1 or maybe 2 will go right and the rest down the middle. Hopefully your strong units get to the mobs instead of your speedy ones. The fast ones serve the dual purposes of opening the lock and hopefully taking the bee hits as they should keep running out in front of your slower, stronger dudes. First time I beat it, my ally squad made it to the boss right after it got unlocked and it was easy. The attempt right after that one failed spectacularly. It's still some harsh rng, but doing it like this helped me get my skill ups even if I can't 3 medal it.
When the event started, I couldn't even kill the regular mobs with my strongest squad. I spent time just straight leveling and evolving my girls. I also tried to time the spending of 120 stamina right before I leveled up so I could keep the farming chain going. Hope this is somewhat helpful.
Edit: Also don't suicide your last squad with no health to the boss out of desperate anger. Retreating will still net you the gold and seeds and some experience.
Edit 2: Just spent 15 minutes setting up a team to try to 3 medal it. Moving strong units around, distributing items to those that needed them more etc. After all that I launched the mission with the completely wrong team
![Mad](https://harem-battle.club/images/smilies/mad.png)