RNG for the card to bring out the specific skill you want
RNG for the card Uma to actually show up during the stats pick
RNG for the card Uma to actually proc the event
RNG for the card Uma to discount or unlock that specific skill
RNG for the card Uma to continue showing up until you have a max discount
This game is RNG hell. I'm not even mentioning other RNG events like your Uma randomly being on their period and dropping their mood.