Outworld Devourer can counter most of the standard solo mids in the game. This guide should help you do just that. Remember that there is no one true build for any hero in the game, and, while I did try to put up alternatives for different possibilities, this build won't work all the time.

Play this hero like a lane bully: harass, steal intelligence, and generally make your enemy afraid to come near. Later, set up kills with your W. Later still, disrupt teamfights with your Aghanim's upgraded ultimate.