Let's pretend that your advice to Achilles and Agamemnon is to "never attack the city of Troy because it will end in disaster". You know that they will not listen to your advice, so what do you do? Well, you could make sure that you give your advice in a very public way so that everyone hears it. The more people that know about your advice the better! In this case your "plan" is to show everyone other than Achilles and Agamemnon that you know what you are talking about and that you give wise council. Then, after Achilles dies at Troy and Agamemnon dies on the way back from Troy, you will be acknowledged as a person who gives good advice.
Odysseus is saying that you must try to out-think people when they don't do what you want. He is saying that there is a way to gain an advantage even in the most difficult circumstances.