What part of the story? I remember playing through for the first time, and getting to that final stage battle (Lake of Black Tears) there was a pretty big difficulty spike.

Otherwise I’m not sure how to help. Keep spamming units, and keep doing support solos. And definitely upgrade your guitar, there are certain skin(s) that “make solos more powerful” and they give a pretty good boost to the different effects. Also go find more solos if you don’t have them all; iirc some of them are kinda optional. Being able to summon The Beast (the car) is a good strategy too, cause you can mow down mobs and the weapons are pretty decent on it too, with the sideburners being pretty op from what I remember.

This is all from memory though, and it’s been over 10 years since I’ve played it.