I think I just figured out the secret to Ephermerality 137 at long last.
While playing stage 3 randomly:
Me: So, Ephermerality 137 always screws me over because of those bottommost familiars exploding late, right? But I can't keep up with them because I have to shoot Keine focused.
...
Who's to say I can't just shoot unfocused the whole time, take out the familiars and let my homing damage Keine?
Suddenly I capture it and I perfect Keine...or I would, had my overconfidence not made me slam into a random bullet on her opener. >_< It's still not easy since I'm not good at unfocused dodging, but to finally have a plan is good.