Dailies are your best bet. Soon as you can do the most stamina-costly Ember dailies, that's where you'll get your best exp - pretty much exclusively gold cards. It's also good player exp, and should level you up pretty quickly. Which means higher team cost, which means bigger team, which means better ability to take on the high-end Ember dailies...
In a couple of days of farming that level, I got enough exp to level all of my Servants to at least their first Ascensions. All of them. Second, in EMIYA's case. From my 4-stars down to my 1-stars, every single one. (I haven't Ascended all of them, but they're now Ascension-ready, at least.) It's as quick a system as you could ask.