Ah! There's my misunderstanding, I think. The first boss said "Let's see how long you can hold out", and I defeated it roughly when its shield was the diameter of the remaining play area, so I thought the boss was survival and died to the walls. So when I got triple shot in the next level, I hung back, only hitting the boss with one projectile, not dealing enough damage to take her out despite a constant barrage.
I'll try a wiser strat next time. Thanks for the tips!