As tenhou said basiclly keep grinding. I think its several hours of game play before you actually catch up to your stamina (less if you only use 3x).

My main complaint about this is that unless this event has a really high drop rate or something its pretty hard to get 600k last i recall from previous events while compared to pps thousands of people likely got the highest reward for Osawari's stuff, which should also help you to make a decent starting group vic.