There are various Lingo strategies out there, including dictionary-like attacks where the goal is to use your first two guesses to include as many distinct letters as possible. A "smart" opponent might have a larger or more comprehensive list of such "starter" words. Difficulty might, in fact, differ only by how "intelligent" the first two guesses are, and then you could use a general strategy after that.

Another factor that a "smarter" algorithm might use things like word frequentcies and letter frequencies to guide guesses. An "easier" opponent might have more of a random selection to it, whereas a more difficult opponent might put more weight on these frequencies.