Originally Posted by Unregistered

SR-Keys are super rare so almost only whales open SR-briefcases. Most SR-Briefcases dont get opened.
R-Keys are not so rare so f2p should be able to get a some of them every day. Many R-Briefcases get open if you got good friends.
N-Keys are good and if you got 1000 of those you will not get N-Keys form the N-Boss anymore.
All N-briefcases get opened if you got many friends.

The SR-girl from briefcases is common in R-briefcases so it is a easy to finish 1x SR daily quests and 3x SR daily quests.
Just use 10-20x N-Gachas to finish 3x N-girls quest and 2x R-girl daily quest first.

That's pretty much what i did. Farmed a bit of ex cards with them. Would still be cool if it leveled other SR cards as well , like a gacha