The phrase "a fierce one" is correct and usable in written English.
It can be used to describe someone or something that is intense, strong, or aggressive in nature.
Example: "In the competition, she proved to be a fierce one, outpacing all her rivals with determination and skill."
Alternatives: "a fierce competitor" or "a fierce individual".
Considering the combustible relationship between the new nations, the rivalry is a fierce one, and the matches tend to take on a significance greater than game itself.
In the 94th minute Graham Zusi finally got a shot on target in a playoff game, and a fierce one too, only for that rarest of events to be met by the rarest of saves — a reaction fingertip over the bar from Nick Rimando.
The world she enters is a fierce one of tacit teenage codes and continuous physical scrutiny.
Whatever game these women play, though, it is a fierce one (and they play it admirably).
She seemed to see Pu Yi more as a father, although rather a fierce one who would occasionally beat her.
For the Wright brothers, argument was the family trade and a fierce one was something to be savored.
Beyond the lacklustre battle for votes this Sunday, there is a fiercer one over the vote monitors, as the opposition under Alexei Navalny backs a boycott and the Kremlin seeks legitimacy.
Progress with this team is rather difficult to establish in what might be a tight group but not a particularly fierce one.
The race for the lone Congressional seat in North Dakota has been an unusually fierce one as Representative Earl Pomeroy, a Democrat, seeks a 10th term.
Already this year, a particularly fierce one for fires, dozens of United States Forest Service firefighters have crossed into Mexico with special clearances to try to control fires before they reach the United States.
The exchange about Ahmad followed a pretty fierce one right at the start of the City AM debate: Khan expressed his disappointment at what he considers Goldsmith's negative campaigning; Goldsmith responded with a list of decisions Khan had made in the past, including about cases he'd taken as a civil rights lawyer, which he contended call Khan's judgment into question.
