Not really true. There are 2 random nodes there. One for chest (2* girls or ampies?) with 2 girls able to pass it and thus 75% chance to succeed. One is pest and you can have up to 3 girls there thus 87.5% chance to succeed. So you can do a one or few runs with 1-1-3 set-up to get 3 medals (moving pests don't count towards that). You will have 87.5% chance to get 3 medals (I actually got 3 medals on first try with that set-up). Then you go 3-1-1 to maximize chance to get all chests (75% chance for all chests and still 50% chance to get 3 medals). There is no RNG involved with dumplings or life crystals (apart from possible bonus drop in that RNG chest).
Technically you can also get life crystals from daily quests hehe. 10 stamina is best to farm if you care for exp/stamina ratio, 40 stamina is best if you want a balanced map with decent exp/stamina and life crystal drops. 80 stamina has slightly better life crystals/stamina ratio, but has worst exp/stamina.