Thank you for playing! The sooner you fully destroy enemy waves, the more time you get to beat the boss. Triple shot is great for destroying enemies, but requires some strategy when you use it against a boss - getting as close to them and having all 3 shots hitting will deal monstrous damage. If you're having trouble then you might want to keep the default weapon, it has the same damage output as if you were hitting 2 bullets of the triple shot at a time. Good luck!
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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!