idiom which would mean someone has perfect command of the cello

Solution 1:

First of all, your sentence works just fine in English. You can, however, polish it up just a tad by removing the second mention of the subject in the second sentence, like so:

The cello is a very difficult instrument. The cello is like a wild horse but is completely tame in his grasp.

Note the word 'it' has been removed. The sentence sounds a bit cleaner now.

Second, there are a number of ways you can express an artist as being of the highest ability.

We can describe them as artists of consummate skill. Such nouns that reflect this thought regarding musicians are: maestro and virtuoso. Note that maestro is wholly specific to music, especially classical music, whereas virtuoso is specific to the arts in general but is often used even more specifically with reference to music.

In your case, with reference to the taming of a wild animal, there are number of synonyms we can use:

gentle - Make an animal docile by gentle handling.

break / bust - To tame a horse.

reclaim - To tame a falcon.

subdue - To bring under control, with emphasis on the exertion of one's will.

The word master works well in your case because it serves double purpose as expressing one's ability as being at the highest level as well as one's command over their instrument. This word is, however, broader than maestro and virtuoso and hence applies to a much wider range of objects. The same goes for the verbal analogues [gentle, break, bust, reclaim, subdue]. Master is a broader term than any of these. So you will want to decide which expressions to use given such factors as precision and connotative meaning.