When calling Mermaid Idol, Flute, be sure you have enough rear-guards for her skill to activate.
Remember that, as a rear-guard, Flute counts herself for her skill.
Having Flute and PR♥ISM-Image, Rosa in your deck, along with a vanguard with a limit break, allows for consistent high powered columns.
If you have units with a skill that allows you to return units back to your hand, be sure to leave enough rear-guards to keep Flute's power high. If that isn't an option, an alternative is to attack with Flute first.