Unity of essence and multiplicity of possible effects are different claims. Multiple creatures do not require multiple independent deities. A single willing and powerful creator can bring about diverse contingent effects.
Rain, vegetation and human beings are distinct created realities; their variety does not entail parts in the creator's essence.
The key response distinguishes the necessary operation of an impersonal physical cause from the power of a voluntary agent. Numerical oneness of an agent does not logically limit it to one possible act.