Don't give up! It took me three days to get the Ryuryu while both Bernice and Dorothy dropped within first three-five runs. So you too will see the completion eventually! Hopefully...
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Gotten Vincent within day's worth of stamina too. All it took for last drop to appear was bringing different team :p
Really, if map gets you mostly the same drop, try clearing it with different team composition. Wiki-goers have half-serious theory that Aigis's RNG uses you team lineup as a seed in some way, so who knows, it might help ;)