"fierce" is correct and usable in written English.
It is an adjective meaning intense or aggressive. You can use it in many contexts such as describing an intense emotion or a situation. Example: The crowd cheered fiercely as their team won the championship.
Exact(60)
It's fierce".
You're fierce".
He's fierce.
Be fierce!
Localism fierce".
Quietly fierce".
Resistance was fierce.
The blowback was fierce.
Competition will be fierce.
But competition remains fierce.
A fierce row followed.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com