Also, if you didn't know already, if you have multiple of the same card, using one to train the other increases it's Super Attack level as well as jut giving exp. But they have to be identical cards, not just the same character.
So, it's worth doing "Seriously Serious Fight" over and over again?
Yeah, you can get multiple Gohan cards by repeating, then feed them into each other to increase the SA level. You get one guaranteed drop on each difficulty level, but after that they're random. Drop rate increases with difficulty level, but Z-Hard Bojack hits stupid hard (30k+ damage super attacks when blocking with AGL types), so it's probably better to focus on Hard. Get Gohan to SA10 (or however far you can be bothered), then Dokkan Awaken him and his SA level carries over.
It's worth doing the "Epic Showdown" event and farming the TEQ R Goku from it too. He Dokkan Awakens into AGL SR Goku (Angel), and his Super Attack deals "extreme damage" instead of the usual "huge damage". Don't think that event is going away again though, so that can wait until after you've finished with the Bojack event.
Also, it may be worth farming enough Dokkan Awakening medals from the Bojack event for Bojack too, even if you don't have him already.
Being an SR, he's gonna have a decent chance to show up in multi-summons when any guaranteed SR stuff is going on, so having the medals ready might be worth it.
He needs 12 of each medal, except the Bojack ones. Instead of the Bojack medals, he needs 12 Gohan medals, and those drop from Seriously Serious Fight. They're the more common drop from it instead of the card.
EDIT: Forgot someone made a wiki for the game now.
Here's the event page, with possible rewards listed.
http://dbz-dokkanbattle.wikia.com/wiki/Galactic_Crisis_Super_Guy_in_the_Galaxy
And here's Bojack's page with the medals he needs.
http://dbz-dokkanbattle.wikia.com/wiki/Galaxy's_Most_Evil_Bojack
EDIT EDIT: The boss fights that drop the Dokkan medals can drop 2 medals on Hard and up to 3 on Z-Hard. Also, the gold awakening capsules on the ground in those areas will be 1 of the awakening medals (same as the boss on the stage), but you can only pick them up once each; they don't respawn next time through.