Damage dealt with Ice refills Cirno's motivation more than with normal shots. Consider refraining from killing enemies that would otherwise be hit by an Ice Barrier or Perfect Freeze.
Enemies in Hard and Lunatic release aimed streams of bullets when defeated. These can be useful to connect separate patterns of bullets when using an Ice Barrier. Try selectively killing enemies before or during the effect of an Ice Barrier.
They can also help refill your Ice Power quickly if you graze them.
Remember that just hitting enemies helps refill Ice Power. Consider not killing them completely if their shots are useful or if you can hit them with Ice.