When you want to promise certainty or assurance, "guaranteed" is a solid word to use. However, if you're looking for some synonyms to spice up your language, you might try words like "ensured," "verified," or "assured." These words convey the same sense of reliability, but with a slightly different connotation. "Ensured" sounds more protective, like something being secured against danger. "Verified" implies a rigorous process of testing or checking to confirm that something is true. "Assured" creates a sense of confidence and trust, as if the person making the promise is personally invested in its success. All of these words offer different shades of meaning to add depth and nuance to your language.