The phrase "a frightening level" is correct and usable in written English.
It can be used to describe an intensity or degree of something that induces fear or concern.
Example: "The storm reached a frightening level, causing widespread panic among the residents."
Alternatives: "an alarming degree" or "a terrifying extent".
Almost four years ago, I went to my GP, Jonty Heaversedge, because the anxiety that had dogged me all my life had reached a frightening level.
A crisis that could have been managed through a democratic process has now escalated to a frightening level of polarization and violence.
The Duchess of York The Escape Since your relationship with the Queen's second son hit the rocks, media scrutiny of your life has intensified to a frightening level.
Instead of the rogue Russian submarines of the cold-war years, several thrillers due to be published over the next few weeks feature a frightening level of day-to-day surveillance, a squeeze on civil liberties and inexorable government creep.
On April 23, AT&T Wireless reported that 44% of its customers were quitting its service every year, a frightening level of churn even in an industry known for its fickle customers.
…but, while the number of people who truly grok the ads might be small, those fifty people also happen to have a frightening level of disposable income, along with the commensurate level of youthful naivete required to blow it all on mid-range vodka and woolen beanies.
The Secret Service doesn't like to talk about these things, but I'll bet that in years to come we'll hear about a truly frightening level of threats during the Obama Presidency.
Nicholas Ray and his screenwriters transformed the teacher's rage into a form that, for Ray, was altogether personal, and ratcheted the violence up to an even higher and more frightening level.
Mr. Valentine, a compact 60-year-old with an intense gaze and a frightening energy level, made his money investing in Jiffy Lube, Mosquito Magnet and LoJack.
Rather than being forced into adopting another bird's young, they escalate the conflict to a frightening new level.
