If you want to make it really simple, take the Narita Express to Tokyo Station, spend the night in one of the hotels in that vicinity, and hop onto an early Shinkansen the next morning.

I don't know where in Hiroshima you have to be, but the Shinkansen will take you to the center of the city.

Or, as others have said, fly to KIX on UA, take the Haruka to Kyoto, spend the night in the vicinity of that station, and enjoy a significantly shorter Shinkansen ride to Hiroshima the next morning.