"staunchly loyal" is correct and usable in written English.
You can use it to describe someone who is extremely loyal. For example, you could say, "My husband is a staunchly loyal supporter of his favorite football team."
His former co-workers and clients remain staunchly loyal.
Throughout her life, Mrs. Steinkraus displayed extraordinary courage, individuality, generosity and taste, and was a staunchly loyal friend.
Muna, whose father died in one of Qaddafi's most notorious prisons for dissidents, loves Omar, the wealthy son of a general who is staunchly loyal to the old regime.
Tribes still supply much of the manpower for Jordan's security forces, and in tacit reward for such patronage have historically remained staunchly loyal to the ruling Hashemite family.In any case, Jordan's legislature does not have much influence.
Both men say the older generation – at home and abroad – are more likely to be staunchly loyal to the government while a newer, younger movement has begun to agitate to for change.
As Tillie, the Drayton family's staunchly loyal housekeeper, Ms. Sanford displayed a defiant acceptance of position and disdain for interracial marriage that stood in stark contrast to the presumed liberalism of the characters portrayed by Katharine Hepburn and Spencer Tracy.
So on Monday, that very specific order from Trump: Declassify precisely those pages of the application for a warrant to surveil former Trump campaign adviser Carter Page that were mentioned in a news conference held by 12 staunchly Trump-loyal members of the House in September.
The introduction of a league-wide wage cap is supported by 41% of fans and it is most staunchly backed by football's most loyal demographics – 30 to 34-year-olds and season-ticket holders.
The original core included avowed fascists, former members of the Vichy government that had been loyal to Hitler, anti-Jewish zealots, anti-immigrant nationalists and staunchly conservative Catholics.
Broderick was staunchly antislavery.
Others looked staunchly defiant.
