I'm also stuck on this quest and Support or not, I can't get past the first node using the standard formations. Someone always get one-shotted by an enemy flagship or three. The only way I've mangaged to get past the first node is using the temporarily available Vanguard formation - and even that is a major RNG show.
If it weren't for that fucking offroute just before the boss node, this wouldn't be nearly as a frustrating bucket sinkhole as it is.
Edit:
Vanguard and support seems to be the best way. Even if your support only downs a single ship, it might be just enough for your own fleet to pass. Just barely managed to S-Rank it just now, right on the hair.
Just... keep trying, keep praying and keep the buckets coming.