C.K

Avoiding damage is more important than getting hearts, unless you're confident in your current HP totals really close to filling a Doki-meter, and the current attack isn't bombs.

Be aware that normal Flirts are more effective if fewer party members are active. Kris and Susie can flirt for two dokis each if Ralsei has been fluffed, Susie flirts for three if she's alone, and the same applies to any combination but that's probably likeliest. It's usually not worth letting Ralsei get KO'd on purpose, but...it's good to be aware of your options.

Get the TreeCake if you haven't. Big group heals are always handy. Maybe also grab some flower sodas, since those heal and give TP.

Start each phase with Kris defending, Susie flirting, and Ralsei LovelyBoyIng. That's pretty much always the best thing you can do with your limited resources at that point, unless someone's dying.

You don't need to rush through the tube. Keep moving forward, but don't lose your head; advance slowly and deliberately.

For the three-string attacks, you mostly don't need to move from side to side. If you stay in the middle, you can just practice the timing of when to jump between strings. (I'm pretty sure the patterns are 100% consistent.)