having no purpose because there is no chance of success (Oxford)
(of actions) having no effect or achieving nothing (Cambridge)
Their efforts to revive him were futile. (Oxford)
It would be futile to protest. (Oxford)
All my attempts to cheer her up proved futile. (Cambridge)
futile efforts
futile to protest
futile attempt
Vain
Unproductive
Fruitless
Productive
Effective
Futility
Futilely