'staunch critics' is correct and usable in written English.
You can use this phrase to refer to an individual or individuals who are extremely critical, firm in their views, and unlikely to change their opinion easily. For example: "The proposal was met with strong opposition from the government's staunch critics."
He also criticized the French and American ambassadors, who have been staunch critics of the government crackdown, in which at least 2,900 people have been killed since the protests began in mid-March.
Many Republican senators have been staunch critics of corporate welfare.
Tea Partiers are staunch critics of immigration policies that, in their view, favour liberal pressure groups at the expense of "mainstream" America.
It might also be welcomed by many East Timorese, even such staunch critics of Indonesia as José Ramos Horta, an exiled leader, and Xanana Gusmao, the jailed commander of the territory's armed rebels.
The growing 10,000-strong paramilitaries, financed in part by wealthy cattle ranchers, were staunch critics of the peace talks held with FARC by the departing president, Andrés Pastrana, which took place with no cease-fire in place.
I also watched with amusement as even the most staunch critics benefited from the show, because many of them got to go on the news or other forums and condemn the sensationalist title and hyper-dramatic presentation of Eaten Alive, and then talk about everything from snake biology to the poor and desperate state of nature programming.
Ben Bradshaw, the culture secretary, has been a staunch critic of the broadcaster in the past.
She remains a staunch critic of Marine Le Pen's far-right Front National.
A staunch critic, Senator John McCain, Republican of Arizona, did not ride the train to New York.
A staunch critic of China, Mr. Nakagawa frequently warned of its rising military might.
She is a staunch critic of US intelligence methods and criticised the National Security Agency's surveillance of leaders of US allies.
