Is "fight windmills" ( In meaning of fighting imaginary enemies) idiom common in modern spoken English? And what is the modern equivalent for the idiom if not.
"Tilting at windmills" is a literary English idiom that means attacking imaginary enemies.
The expression is derived from the 1605 novel Don Quixote by Miguel de Cervantes, and the word "tilt" in this context comes from jousting or tilting: A combat or encounter (for exercise or sport) between two armed men on horseback, with lances or similar weapons, the aim of each being to throw his opponent from the saddle (OED).
Related idioms include going on a wild goose chase and chasing rainbows. All three phrases make the point that an objective is illusory, impractical, or impossible. As such, people who tilt at windmills, pursue wild geese or chase rainbows are frequently said to be 'off/away with the fairies' and 'in a world of their own'!
"(You're) fighting against the wind." or "(You're) Fighting the wind" This means that fighting is futile; has no point to it.
"Pissing against the wind." When you piss against the wind, the wind blows your piss back on you. So, in some cases you're fighting something that will just hurt yourself.