The phrase "tilt windmills" is often used to describe someone who is fighting an impossible or futile battle. However, there are plenty of synonyms to express this same idea. Some of these include "chasing rainbows," "banging your head against a wall," "beating a dead horse," and "fighting a losing battle." Other similar phrases could be "running up against a brick wall," "pushing a boulder uphill," or "swimming against the tide." No matter the phrase used, the sentiment is the same: sometimes we have to accept that certain battles are unwinnable and move on to more productive endeavors.