The phrase 'immensely serious' is correct and can be used in written English.
It is used to describe something that is very serious or important. For example, "This issue is immensely serious, so we must take necessary steps to address it right away."
Cameron is an immensely serious and able guy".
Coulter turns out to be an immensely serious, erudite craftsman, highly self-conscious about his effects.
I take the problems that he has set himself about the nature of the crowd as being immensely serious.
Mitchell is immensely serious - until she hoots with laughter, when she realises she has just said something more than usually earnest.
She added: "All of them really deal with immensely serious and in some ways quite dark subjects, but the luminosity of the writing makes them more than bearable.
He was capable of enormous wit, and his writing is suffused with a memorable sense of humour, a lightness that seduces us into his immensely serious topics.
He was immensely determined, deeply serious about his mission in life, and discreet - even secretive - by nature.
"They [Cranston and Hart] became friends from working together, so that has helped immensely". While staying serious may have been difficult at times for Hart, Cranston's challenge has to do with the mechanics of acting, setting aside his performing instincts while playing a person limited in motion.
And yet in spite of these serious flaws the immensely vengeful, tedious, depressing, obsessive, paranoid, superstitious, reactionary, authoritarian, misogynistic and antisemitic flavour of the Ayatollah's thought shines through.
Though the epigenetic mechanisms are themselves quite simple, easy to explain and reproduce, the complex network of interactions which they induce in order to produce a unique pattern of gene expression, is immensely complex and poses a serious challenge for current and future research in the field of epigenetics [ 7, 8].
His Keating is playful, serious-minded and immensely sympathetic.
