"greatly entertaining" is correct and usable in written English.
You can use it to describe something that is particularly fun, enjoyable, or funny. For example, "The movie was greatly entertaining and kept me on the edge of my seat the whole time."
But when taken on its own stylish terms, "Flatliners" is greatly entertaining.
As a huge fan of her movies, and as a chef obsessed with the minutia of moviemaking, I found her description of the rituals she employs to stay sane during the process to be greatly entertaining.
Therefore, due to the fact that animation and whiteboard drawing videos utilize this drawing aspect, they are greatly entertaining.
It is a depressing story, without any obvious remedy, but it is only half this greatly entertaining book.
Meticulously described, and greatly entertaining to read, the sheer pleasure-value of Bond's myriad "games" should not allow us to overlook their deadly seriousness.
There are also some wonderfully surreal passages of when the Beatles were let loose amid the splendours of their Plaza Hotel suite, where they were minded by an eccentric but greatly entertaining New York disc jockey, Murray the K.
You have greatly entertained us with the tale of the miserable life you led when married to Sir Macca.
You can have a perfectly good one-night stand, be greatly entertained, and still not know the other person when it's done.
I won't critique anything contained within the video — it appears to be a school project, after all — except to say that I watched all 8 54 of it and was greatly entertained while actually learning something at the same time.
I would have added that I was greatly stimulated and entertained by your last novel ["Zuckerman Unbound"], and that of course after three decades I understood perfectly well what you were saying about the writer's trade — how could I not understand, or miss suffering the same pains.
In this neither entertaining nor enlightening manner, of course, the debate greatly resembled, well, the debates of the pre-YouTube era.
