This seems impossible to translate properly - they're making a pun on コーラス / 凍らす, both are read the same (koorasu) but the first would usually mean "chorus" in the musical sense, and the latter "to freeze". So Marisa's question can be understood as "Who has the best choruses?" or "Who's the best at freezing?".
Cirno!Wrong!Which stage 2 boss has the best "chorus?"
See comment belowUm... That'd be... Mysty-chan...Huh? Did Cirno have that ability?Goodbye.Interesting.Because she's frozen.