"fierce assertion" is correct and usable in written English.
You can use it when you want to emphasize a strong statement or opinion. For example: "Her fierce assertion that everyone should take responsibility was met with much support."
A supplication to Scandinavian royals might not seem to indicate a fierce assertion of island autonomy.
In the end, she has it all, as the Victorians might have perceived it, and the novel, published in 1847, has endured as a kind of feminist ur-text, richly analyzed, sociologically and psychologically, for its fierce assertion of womanliness.
Gerald Barry's starting point was a prosthetic right arm exhibited in the medical gallery that had been fitted to a pianist at the turn of the 20th century, its thumb and little finger spaced so that she could play octaves; his charmingly slight piece began with rocking Feldman-like chords and ended with a fierce assertion of the octaves that this anonymous pianist played.
The track serves as a fierce assertion of sexual confidence that sweeps haters to the side as Cakes chants, "Get the fuck out my face".
They make no violent assertions.
President Obama's increasing reliance on armed drones to kill terrorist suspects has sparked an international outcry and a fierce domestic debate about the government's assertion of its right to kill in secret.
The growing assertion of the Christian south is provoking fierce doctrinal arguments, too, often over their preference for literal readings of the Bible and a conservative view on social issues.
Mr. Klaus disputes assertions that human activity is to blame for global warming and is a fierce Euroskeptic.
The assertion has resonance.
No assertion goes unqualified.
"Proof, not belligerent assertion.
