Have you tried simply combining all the best removal spells with the best fliers in the format? Ashcloud Phoenix, Butcher of the Horde, Stormbreath Dragon, Sarkhan, the Dragonspeaker.I can't seem to come up with a good Mardu deck which is frustrating.
An article on SCG that I recently read mentioned something about there being no good 1 or 2 drops, and the solution to that was to simply not play them. This means you run taplands, and your 3-drops are simply removal spells. Arc Lightning, Anger of the Gods, [card]Hero's Downfall[/card], Banishing Light, etc.
The other end of the spectrum really just looks like Mardu Tokens, featuring Mardu Ascendancy, which saves your ass from the lesser sweepers.