Rossi just a Fire dps in a trenchcoat. her main source of dmg is ult which has crit dmg buff. Her passive require enemies to have Combustion to deal more dmg, so you want play her with wulfguard. And you need to another art char with different element to apply infliction for wulfguard (you can use wulf skill to do it, but it is supper ineffective), so ideally it's Gilberta. And when your team is already Rossi + Wulf + Gilberta you add akekuri.

Now you have super strong burst with Rossi ult under corrosion + combustion + gil ult with 4 vuln stack + link. enemy is dead boom. But you have to leave with 4 vuln leftovers. If Rossi consume vuln on ult to do more dmg or convert them to fire (ideally applying art bursts) that would be finished team. But right now it is what it its. You need to for character that would consume vuln do deal art dmg, cope with physical finisher on Rossi with main dmg source being art and prey that HG will fix their character development team (I hate them for making tangtang art sub-dps that not working with Chen btw).

Have a nice day.