Masa&Mune is tough enough, until you're well into your fifties, I'd say, but since Spekkio stays in that form all the way up to lv.98, there's plenty of time to fight him in New Game +. The Red Nu on the other hand, is just a plain arsehole of a fight, unless you go for the Frog Flare tactic mentioned up on the front site, and even then, it's still far from easy.