Most of the games usually have around level fifty to sixty mons with the elite four or champion so unless you're that willing to go insane grinding, I'd say sixty is the safest minimum level. There are some that go over sixty like Cynthia but sixty is just the safest. Altho you're right, DIantha is an exception I guess but then again, xy isn't exactly that difficult considering you're given two of the best megas for free and this game has a huge variety of pokemon to choose from