Megahorn could be useful for Lucian (Psychic). There aren't any grass-types in the E4 (besides Cynthia's Roserade, which is half-poison, and a fire move would kill her anyway), and the only trainer who you would have better coverage against is Flint (Fire), assuming you don't have Bounce or would rather not use it.
Team-wise, I'd say bring a fire- or flying-type to take care of Aaron (Bug) and Cynthia's Roserade (Fire would also take care of her Lucario), a water-type or two for Bertha (Ground) and Flint, an ice-type for additional coverage against bertha and for Cynthia's Garchomp and Togekiss, an electric-type (prefferably physical, like Electivire) for Cynthia's Milotic and Togekiss, and any other pokemon you may need to take care of Cynthia's Lucario and Spiritomb, Lucian's Bronzong, and any other pokemon seen
here that you may have problems with.
(This is all assuming you're playing Platinum, by the way.)