Hello Mohammad! The phrase "nothing short of" is used to emphasize how great or extreme something is and it can be used in just about any situation - not just in formal situations (in natural English as well). As far as I know there are no exact synonyms for this phrase but you could write your example: "What he has done is nothing short of amazing.", like this: "What he has done is absolutely of amazing." or "What he has done is unequivocally amazing." "absolutely" means definitely and "equivocally" means leaving no doubt or without a doubt. I hope that helps!

Thanks Matt!

You can use definitely use this phrase informally also. The above sentence would sound just fine in the spoken language. If I wanted to be even more informal, I might say "What he has done is simply amazing."