'more fierce than' is correct and usable in written English.
You can use it when comparing something to something else to show that the first thing is more intense than the second thing. Example: The summer heat this year was more fierce than last year's.
He was supposedly going through a separation and he looked more fierce than ever!
"When I was in the United States, their highway police seemed to be more fierce than Thai police.
But the rain last night was more fierce than ever," added Deng, who reportedly dodged death by hiding under a pole.
There is perhaps no competitor more fierce than Shelley Ross, the executive producer of "Good Morning America," which has been no slouch in morning event programming either.
Retail chains selling multiple varieties of phones and network packages blossomed along high streets in the 1990s, but with margins squeezed and competition more fierce than ever, network operators have an incentive to cut out the middlemen and sell through their own outlets.
Intel will meet a competitor far more fierce than AMD in Qualcomm.
Chelsea's putting on more than a brave face: it's more fierce and exciting than ever, even as the Lower East Side maintains its once-subversive, still surprising edge; elegant Upper East Side aeries groan under marquee names; and SoHo fights mall status with a new and refreshing energy.
Competition for headlines, more fierce in Britain than in the United States, accounts for a lot of the emphasis here on shock art.
Moreover, this discrimination has itself been unequal — more fierce against some groups than others and more pervasive at some periods of history than in others.
This ongoing struggle was at no time more fierce — or more consequential — than during the Civil War.
The moves by governors, legislators and citizens' groups have been spurred by judicial contests that have been more fierce and more expensive than ever.
