When looking for synonyms for the phrase "make weaker," there are a number of options to choose from depending on the context of the sentence. "Diminish" is a strong option to use when discussing a reduction in strength or power, while "weaken" is a broad term that can apply to physical or emotional weakening. "Softening" can refer to the process of making something softer or less intense, and "diluting" is often used in reference to liquids or substances that have been watered down. Finally, "debilitate" conveys a sense of causing someone or something to become weaker or less effective over time, to the point of losing function altogether.