The phrase "astounding achievement" means accomplishing something remarkable or extraordinary. It refers to a great success that leaves people amazed or impressed. Full definition
Regardless, AV is a truly astounding achievement for one man (basically). (blog.us.playstation.com)
Do not downplay accomplishments, but phrase them humbly: «I was very fortunate to be able to [insert astounding achievement here].» (inc.com)
When Walt Disney World opened in 1971, it recreated the already innovative and revolutionary Disneyland on an even grander scale, presenting to the world one of the most astounding achievements in construction and technology it had ever seen. (dvdizzy.com)