How does one respond to those who say that everything has a lord, such as death, rain, and vegetation, and believe in the multiplicity of gods?

This belief is disbelief (kufr). The Holy Quran has demonstrated with conclusive evidence the falsity of there being another god for the universe alongside Allah Almighty, among which are His (Allah's) sayings: ﴿Allah has not taken any son, nor has there ever been with Him any deity. [If there had been], then each deity would have taken what it created, and some of them would have overcome others.﴾ [Al-Mu'minun: 91], and His (Allah's) saying: ﴿Had there been within them [the heavens and earth] gods besides Allah, they both would have been ruined.﴾ [Al-Anbiya: 22]. Every atom in the universe testifies to the Oneness of Allah and His uniqueness in creation. A Muslim who is not qualified to debate atheists and those who promote falsehood is advised to avoid them, fearing that their misconceptions might seep into his heart.