The opposite of un-achievable is achievable, as something that is achievable is capable of being accomplished or attained. Another antonym for un-achievable is attainable, which means something that is achievable or accessible. Feasible is another antonym for un-achievable, which means capable of being done, accomplished or carried out successfully. Possible is another antonym of un-achievable, which means something that can be done or achieved. Finally, reachable is another antonym of un-achievable, which signifies something that can be reached or achieved. These antonyms provide a range of possible alternatives to the word un-achievable, giving us a clearer idea of what could be attainable or accomplished.