Learn English – Someone who thinks they can do everything but they can’t

A term with some topical currency which is unlikely to be misunderstood even after that currency fades into the background is 'Major Blowhard'. If you have sufficient control of emphasis and intonation, you will be able to convey a great deal of negativity speaking the phrase.

The term gets its currency from the recent (2015, San Francisco) success of a play called "The Braggart Soldier, or Major Blowhard". Originally written by Plautus (and titled Miles Gloriosus), the contemporary highly praised production was adapted and directed by Evren Odcikin from a translation by Deena Berg.

Unfortunately, the terms 'armchair soldier' and 'armchair warrior' have been co-opted by the realities of modern warfare--present-day verities which may muddy the waters for any term you choose, for the simple reason that drone warfare puts some soldiers in the virtual thick of it while at the same time leaving them safe in their armchairs.

The use of the 'armchair soldier' term in the title of an opinion piece, "Drones, Ethics and the Armchair Soldier" written by John Kaag and published in The New York Times, suggests this ongoing co-option is gaining, rather than losing strength. In any case the term's use has always been weak for your purposes due to its apt application to those who have never seen action, but who imagine nonetheless that they are masters of realtime strategy and tactics.

A special snowflake is someone who feels he is different (read: better) than everyone else.

This term is quite informal and has gained a lot of popularity recently; I often see it used in criticism against social justice warriors. Its origin lies on the oft-repeated trivia that no two snowflakes are the same.