I luckily pulled Feng Yan and Chow about a month apart (my only nat5 pulls, f2p) on my 3rd month of playing so my priority is to auto GB10. I first tried

Vero(L)-Bella-Shannon-Bernard-FY

and it fails. Bella, Bernard, and Shannon are only 5*. When I got Chow, I tried replacing Bernie with him:

Vero(L)-Bella-Shannon-Chow-FY

and it failed, so I was thinking maybe I needed better runes on Bella (he's my worst runed unit in that comp) and 6* him. I retried that same comp after the Bella upgrade and it still failed. So I was back to farming GB8. During the 3rd Anniv event, I was able to 6* Chow and forge him better runes. He's lvl39 atm. So I tried again:

Vero(L)-Bella-Shannon-Chow-FY

and again it failed. So I'm thinking I still need better runes. For kicks, I tried a sample run without Shannon.

Vero(L)-Bella-Bernard-Chow-FY

And it surprisingly worked. It turns out that turn speed is a lot more valuable than def boost. After a few runs I think this comp has a ~80% success rate primarily due to my Vero being slow. My fails always come from not cleansing def break at the boss level. CC was never a problem in stages 1-4 since nat5's are really tanky. Anyways, I know this should've been a cake walk with my lucky pulls but I learned a lot from this experience.