If damage is what you focus on Umbral, then you better don't use her at all. Yes, she has good multiplier and can deal good amount of damage, but she is not in your team for big numbers. Better read her kit again and rethink where is her strenght. 

What is the best way to use her?

High accuracy, speed and defence. She is strong CC champion, but need some investment. If you realy plan to use her, her Provoke must be booked to 100%change, then stack as many accuracy you can get from equpment, get her speed high enough so she can block enemy with Provoke before they act and then go for defence. Frost set works well for her, but remember, stats are more important for her than set bonuses. If you get her good additional stats beyond those, she will do some damage as well. 

I used her all the way from end Bronze to Gold 4 and she is key champ in my second account Arena setup. Used her in some dungeons for waves also, until I got fast and strong teams that can clear waves without her. 

Thanks