I love Pokemon Diamond too! It's sooooo fun! But my favourite game is Mario Galaxy. Anyway, my favourite Pokemon is Mewtwo. No, it's not because it's so good (I have one Fire Red, and it OWNS!), or else Arceus would be my favourite (as my username and avatar would seem to say), no, Arceus is my second. IDK why I like Mewtwo, I just do...
Now, advice on beating Volkner: I think what I did was just use my Dialga, as my Grotle JUST Evolved (I was trainig Staraptor, my favourite non-Legendary Pokemon in Diamond or Pearl), and was at like Lv. 36 or 37... anyway, my Dialga was like Lv.49 or something, and I just destroyed him... IDK how... I think I used Earth Power alot and OHKOed or 2HKOed all his Pokemon... it's been so long since that happened, my memories are vague... but I remember most of the E4, so if you need help on them, PM me!
Now, how to raise a Pikachu: I don't have one in Diamond, but it can't be as hard as raising an Arceus. Anyway, just battle with it tons, also trading it and then getting it back boosts EXP gained, so that makes Leveling up alot easier. The E4 is also a great way to earn EXP, but I take it you haven't beaten them yet, as you are still on Volkner (or are you...). If it is at a really low level, just attach an EXP Share to it, start with it, then bring out your best Pokemon to pwn them. I do this when I want to raise a Pokemon quickly, and, hey, it works for me!