The phrase "astonishing ease" is correct and usable in written English.
It can be used to describe a situation or task that is accomplished with surprising simplicity or without difficulty.
Example: "She completed the complex puzzle with astonishing ease, impressing everyone in the room."
Alternatives: "remarkable simplicity" or "incredible ease".
The cutters sliced the fabric with astonishing ease and exactitude, but I was a little more astonished at how they sliced my finger, three times, without my noticing.
Doc, played with astonishing ease by Paul McIsaac, rarely looks back.
Paire generates pace with astonishing ease, taking the ball early and confounding opponents with his unpredictable and bold shot selection.
His is a cruel universe, created with great sensitivity and handled with astonishing ease, fluidity and economy.
He traveled fluidly on point in his sneakers; his torso arched and dipped with astonishing ease; he wrapped one leg around another in knots.
The spirited Azarkena had superior hands, better footwork and near-perfect shot selection as she withstood Sharapova's blistering pace with astonishing ease.
At the beginning of the stair dance, when Robinson performs solo, he shows us his astonishing virtuosity: ease, precision, variety, musicality.
Indeed, what is astonishing is the ease with which Mr. bin Laden was able to base his first-string holy warriors in the West.
The damning question-Quis custodiet ipsos custodies-is here staged with astonishing confidence and ease, with the threadbare power of a visionary artist taking the instrumentality of his own art to task.
Duncan was my oldest and closest musical friend, a practitioner of astonishing insight at ease, it seemed, in the language of sound, able to offer a musical solution to any problem.
We have grown accustomed to Federer making astonishing shots with an ease and grace that take your breath away.
