The phrase "as hellish" is correct and usable in written English.
It can be used to emphasize the extreme or intense nature of something, often in a negative context.
Example: "The conditions in the abandoned building were as hellish as I had imagined, with darkness and decay everywhere."
Alternatives: "as dreadful" or "as nightmarish".
Kunta's journey on the slave ships to America is as hellish as one could possibly imagine.
The damage to these buildings, nearly lost in the shadow of the World Trade Center, was as hellish as it has been unrecognized.
There is no mistaking the fraying of his film work after the war; no one could sit through the submarine drama "Hellcats of the Navy" (1957), which makes life beneath the waves look about as hellish as a wet kitten, without suggesting that its star — and his co-star, Nancy Davis — seek employment elsewhere.
And life inside Horsens State Prison, the Danish institution, now closed, where the movie was filmed with a cast that includes former inmates and guards, isn't as hellish as the situation behind bars is portrayed in American movies, some of which are so gory they qualify as prison porn.
By 9 30 p.m. 1,500 sweaty people are packed inside, where it's almost as hellish as it is outdoors.
Inside, the gas chamber is as hellish as you'd think.
Denise Lewis as Tina Turner and Davro as the hellish Tom Jones/Fred West hybrid were just the tip of the iceberg.
It retained some of its original beauty, even while large sections had fallen into decay as it served as a hellish warehouse for the poor, according to people who spent time there in the 1980's.
As strange, and at once as luxurious and hellish, as the dining room Rothko planned in New York - a place where, instead of making small talk, diners would be menaced by sensual, occult, claustrophobic paintings.
A fantastic creature with a Tweety-bird head and tiny birdlike beak could be either celestial or hellish, as could a dark-skinned presence of ethereal grace who bares a set of ferociously sharp teeth.
You described some parts of the journey as "more hellish than hell".
