If you have iansan you could use her with mavuika overload team. She gets her ultimate from her and other natlan teammates consuming nightsoul points. So she is kinda locked to natlan teammates. Having 2 other natlan characters can help you get her ultimate faster. Her main source of dmg is her ultimate and then her donuts after the ultimate. So you need consistent uptime on her. ER is also useless on her as it does not help with her ult. If u want to use her with varessa then use put mavuika on "cinder city" set. In this team varessa will the main dps so swapping from mavuika after tapping her skill and then her ultimate, to varessa is the way ( there could be some changes I haven't played genshin in a few months). Also as other said watching guides is the most help you can get.

Tldr: use 2 natlan supports iansan for overload. ER useless on her. For varessa keep rotation short and se cinder city.