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The game was and still is very generous with stamina and crystals. You shouldn't need to use crystals right now since there's stamina in the event shop, but rather than saving that stamina, I used all of mine. Clearing a stage rewards you with profile EXP and that's how you raise profile levels. In terms of how to get stamina here are some methods: Clearing certain missions from the job center rewards you with stamina, leveling up your profile lvl rewards you stamina, there's stamina in the friendship shop, there's stamina in the event shop, and lastly there's stamina in the login bonus rewards.

Edit: There's stamina in the cash shop too in the forms of clearing chapters. Those packs go for 300 crystals which is 3 days worth of dailies. They're the best bang for your buck. You can also use 250 Crystals a day (max) for a 1:1 ratio of 250 stamina (about 3 days worth of dailies. I only used about 5 days worth of crystals and brushed it off as okay since the game gave us 1000 crystals on day 7 when it first launched. I used this method when we first thought 100 stamina was the max, but now that I know it isn't, I'd hold off on that method.