"assuredly a" is correct and can be used in written English.
It is normally used to highlight the certainty of something, usually a fact. For example: "It is assuredly a fact that the Earth is round."
The koala is assuredly a creature of leisure.
But the gay-marriage debate, while assuredly a civil rights argument, is much more than that.
There's no such state tax, but there is most assuredly a local one.
There is assuredly a case to be made for reducing its size and ambitions and giving greater responsibilities to individuals.
But he spoke confidently and strode assuredly – a man who knew things would turn in his favour.
Alfredson is assuredly a keen reader of Le Carré but everything shows he has thoroughly read Joseph Conrad.
He is assuredly an activist, and a bold one.
There is no definite pattern to the crystal array, called grain, and it is assuredly an analog process.
Not a Brexit conspiracy, but assuredly an inspiration for Boris Johnson's fatuous and burbling battle cry of "Independence day!" Now the Tory leadership campaign has begun and, incredibly, movie advertising is again playing its role.
As "South Park" entered its fourth season, I spoke recently with Mr. Parker, 31, and Mr. Stone, 29, about their work, starting from my conviction that despite the flatulence jokes, the show is most assuredly capital-A art.
(And adults too; I had earlier heard a parent assuredly identify a salamander as a snake).
