1.Deidara was 19 at that time and he was on a rampage in his village.Don't forget that Itachi was in ambu at the age of 13
2.Part 1 doesn't count
3.Again...Naruto needed help to solo all you mentioned+at that time he didn't master kurama's power
1. Lol, get your facts straight! Deidara was 19 in part 2, not when he fought Itachi, that was back when he joined the Akatsuki. And considering that he along with the other members where all seen as silhouette at the end of part 1, that must mean he has been a member for at least 3 years, in which case he was max 16 when he fought Itachi. But i'll admit saying Kid Deidara was exaggerating a bit, but the point is that it was long ago.
2. Well why the hell wouldn't part 1 count?? And just for the record Itachi soloed Oro and Deidara pre-part 1 and Kakashi in part 1. If that stuff doesn't count I guess he soloed 0 people, right??
3. Not really, Naruto soloed Kakuzu, 3rd Raikage (At that time he had mastered Kuramas Power), Kiba, Kabuto and Gaara fair and square in 1v1 matches. In the others he either lent some of Kuramas chakra or lost control completely (Against Pain), so if you don't wanna count that then it's up to you. I can see why you wouldn't count Pain, but I'd still count the others at he was still in control and the Nine-Tails has always been a part of his power to some extent.