"immensely hard" is correct and can be used in written English.
You can use it to describe something that requires a huge amount of effort, such as "The assignment was immensely hard and took me much longer than expected to complete."
But as is so often the case, incumbents find it immensely hard to disrupt themselves.
Finding that electoral space is immensely hard, as is governing from the centre-right.
But the sheer democratic force of a referendum result is immensely hard to resist.
He worked immensely hard at Warwickshire, where his mentor and coach was that Edgbaston doyen Tiger Smith.
Sharing such feelings is immensely hard, as many fear being condemned and consequently feel exiled in society.
Long-winded, repetitious, immensely hard to understand, the human genome would not obviously qualify as the ideal weekend guest.
More than that, he was popular and immensely hard-working.
A passionate woman in the best sense of the word, always willing to help anyone in need, she was intensely practical, immensely hard-working and unstinting in her inspirational support for her Goldsmiths students.
Only occasionally did he show his learning as when he was heard on air to describe as "very Voltairean" a politician who spoke of going off to do some gardening but he was a fluent writer, well read, well informed, numerate and immensely hard-working.He was also principled.
Valencia do have strength in depth (last night they had Baraja, Aimar, Pellegrino and Angulo on the bench, compared to Madrid's Borja, Portillo, Juanfran and Mejía), Benítez has managed his squad superbly, rotating cleverly, and Valencia are perfectly organised and immensely hard-working.
In his last speech to conference as leader of the union, John Dunford, the association's general secretary, warned that a culture of instant gratification has made teenagers "immensely" harder to teach than a decade ago.
