Just letting you know that you don't need mythical Pokemon to complete your dex. For the other legendaries, I actually completed my dex of legendary Pokemon just by offering 5IV Pokemon on the GTS. But if that doesn't work for you, just grab one legendary Pokemon, put it up asking for a different one, repeat with the one you get until you're done.
People only come here for help with dex completion when they've almost finished, or are trying to get some really hard ones, like those that evolve at later levels, or are difficult to obtain. Try putting in your own effort and people will be happier to help you.