I can't beat the boss in Extra (event). Maybe, because I'm only 18 rank and don't have so cool cards? What to do? Very want to get just 1 R card. And you say that "play in Extra... it gives you more these 'round pieces' (forget how they call). But if I lose, I don't get anything. So terrible :(
here is my advice from someone who had to deal with this fairly recently: Soul amount on your deck is key. So try and make sure you have maxed out cards in your deck, that you pick someone with a maxed UR to be your support if you can manage it, and use the FP boost that gives you increased soul amount for your bosses
eventually you’ll be able to get down some 1* bosses, and once that’s done you’ll get the balls you need to get the event cards, then just level them, rotate them into your deck, and rinse and repeat.
By the time I maxed out my first event SSR I was able to take down all the bosses with no problems.
Also do some Gachas to get more cards:
http://sb69eng.tumblr.com/post/121306314765/guide-gachas
I made this during the previous green event, so the cards that have raised chances are different now, but this should help.