The phrase "more grievously" is correct and usable in written English.
You may use it to emphasize how something has been or will be more severely impacted than previously, for example: "The recent drought damaged the agricultural industry more grievously than was seen in any other year."
The Greek people would suffer more grievously than now.
Perhaps no other part of the country suffered more grievously during the Vietnam War.
(The lesson here, then, might be that those who fight with Macy's end up wounding themselves more grievously).
Gardner isn't raucously funny, like Harlow, but she's more womanly, and more grievously wounded, her eyes flashing, when Gable rejects her for Kelly.
Yokohama, being nearer the epicentre of the earthquake, was more grievously damaged than was Tokyo; it was badly damaged again by the bombings.
Their father, Hasan, quickly carried one girl to a nearby clinic, unaware that the other lay more grievously wounded under a pile of clothes.
Dozens more are grievously injured and are fighting for their lives in nearby hospitals.
Kajaki's executive producer Gareth Ellis-Unwin – best known as one of the producers of The King's Speech – says Kajaki is "politically agnostic": although one man lost his life, and several more were grievously injured, it's a story of procedural cockup and red-tape as much as anything else.
The overground rail network, which virtually seized up after the Hatfield crash six years ago, is now carrying many more passengers but on grievously overcrowded trains in and around London.
On it was a picture of a child, no more than two or three, grievously wounded, probably dead.
Along the way, the Marines took some of the heaviest casualties of the war: about 360 killed in action and more than 4,700 wounded, many grievously.
