What should I do for next Challenge?

I would honestly forget about trying to get a better Saber. The odds in this game of getting a gold servant are really low. Bedivere is solid, but not worth the investment. There's a welfare 4-star Saber probably coming soon with the Halloween event in Brave Elizabeth, but I don't how good she is in JP currently. Fergus is pretty decent and Caeser can produce stars and buff NP damage. I'd level up those two. I highly recommend you farm Nero Fest for the lottery boxes cause you get EXP cards, mana prisms, apples, QP, gems, and ascension materials. You can also infinitely roll the lottery and keep getting all this stuff. I know you want servants, I've been there man, but do not miss out on events if you can. I missed out on the Da Vinci event when I first started playing and I regret it cause I could have gotten a lot of stuff to strengthen my servants.