There are numerous synonyms for the phrase "strong stuff," each with its own unique connotations. For example, the phrase "potent mix" suggests a combination of powerful elements, while "hardcore" implies something extreme and intense. "Heavy-duty" suggests durability and strength, while "formidable" implies a sense of fear or respect. "Robust" denotes a product that is hearty and well-made, while "powerful" emphasizes strength and force. "Intense" implies a high level of emotion or physical sensation, while "puissant" refers to remarkable force or effectiveness. Ultimately, the choice of synonym depends on the specific context in which the phrase is being used.