The phrase "impossible mission" refers to a task or goal that is extremely difficult or nearly impossible to achieve. It implies that the chances of success are very low or that it cannot be done. Full definition
The most impossible mission is putting someone you love in danger. (ram.org)
Nowadays, it is like impossible mission to accomplish, as it becomes harder and harder to find a boyfriend or girlfriend. (cupid.com)
Thankfully, players won't be alone on this seemingly impossible mission; up to three players can enjoy the classic arcade - style shooter simultaneously on a single console or online. (co-optimus.com)