Frog Flare breaks this game.
Last year I made a thread about team comp. I saw a ton of answers which speaks volumes of how well balanced the cast is. Ultimately, I went with Crono, Frog, and Lucca.
Frog is outclassed physically by Crono and Ayla, magically by Magus and Lucca, and overall by Robo.
Yet when he's paired up with Lucca, you have ostensibly the single most damaging attack in the whole game. You can use this four times with each character - costing 20 MP - before needing to restore their MP. At base - with Frog at full health - this still does upwards 2000 damage. When damaged, it goes well beyond 3000.
It completely trivialized Lavos in any ending final boss fight I faced him in. It trivialized the Dream Devourer too. While I'm all for doing crazy damage using dual techs as Ayla or Robo, nothing was more destructive than Frog Flare.
If Frog needs to heal up, Lucca's Flare does crazy damage on its own. But when they're together, paired with Crono's Frenzy, it absolutely shreds every challenge in the game.