To decimate is to reduce by one-tenth, not to one-tenth. It meant originally to punish mutinous troops by executing one man in ten, chosen by lot. Hence by extension it means to destroy a large proportion; the suggestion it now conveys is usually of a loss much greater than ten per cent. Because of the flavour of exactness that still hangs about it, an adverb or adverbial phrase should never be used with it. We may say "The attacking troops were decimated", meaning that they suffered heavy losses, but we must not say "The attacking troops were badly decimated" and still less "decimated to the extent of fifty per cent. or more".
The following truly remarkable instance of the misuse of decimate was given in the course of correspondence in The Times about the misuse of literally.
I submit the following, long and lovingly remembered from my "penny dreadful" days: "Dick, hotly pursued by the scalp-hunter, turned in his saddle, fired, and literally decimated his opponent".