I just mantain some Stage 3 cards with that rule, the rest i try to rush to 100, specially gacha only cards. I got 2 SR upgrades thanks to that.
And like Coliseum event, remember, Moe cards got SED quickly until lvl 60. If you have N moes, consider them to upgrade first if you don't have slayer cards. Babe cards or R moes are the second choice if you maxed N moes.
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Stuck at 560k and out of money for now, i hope to get some slayer cards before second phase and help. Tenhou did the same with one hit