Level 4 cards
Never use them. They'll just be swept aside as the bosses bounce back and forth anyway. Stick to level 2 spells.
I always manage to somehow lose a life on Aya.
This is too be expected. I'm not sure what mode you're playing or what character, but in general, dying in stages 6 through 9 is very hard to avoid since the AI lasts so long. In general, it's around 3 minutes in the first round, and 2 minutes after you've lost once.
What's very strange though is that you'd find Aya harder than Komachi. Granted, difficulty is subjective, but I think most would say Aya is one of the easiest characters in the game, while Komachi is one of the hardest. As Reiko said, most of what Aya throws at you can be streamed, including the big special bullets. Try slowly working your way across the bottom, then using a level 2 to change direction. You're probably making things more chaotic than necessary by moving around too much.
Yeah, I know they're durable due to algorithms and such, which I hate
Don't hate it, it's the only thing holding the game together!