It really depends on which nature it has. For instance, I have a Haunter from SS that has 93 attack and 165 in sp. attack. Therefore, he'd be a special attacker. I think Serebi has a lot of info on this,
You should EV train Volcrona in Sp. Attack and Sp. Def or Speed
It depends on how you train them, but yes, it is good to have several different types in your party. You should check their IV stats and EV train them as well. Teach them good moves, and they will dominate. Yes, they are good Pokemon if you train them good enough you wont lose often. ( heres a tip Vaporeon doesn't evolve )
How far are you in the game? Victory road always works, or using your VS seeker. Or, you can EV train. You can search the information if you want to ev train. It can boost stats by a big chunk each time you level.
No,to EV train you need to win experience points,try breeding your lv 100 Pokemon and EV train him. I thought it would be informative to add that you can give the level 100 Pokemon 10 of each vitamin in five stats and 1 vitamin on the sixth stat. Sorry but that's the best you can do.
You EV train it. See serebii.net for more info.
of course you can unless the level of the Pokemon is to low to reach 252. just EV train until it wont allow you to give your Pokemon anymore.
Train it as you train all pokemon.
It's pretty obvious... EV train (I suggest EV Training in Defense)...
Yes. The vitamins don't affect how they gain stats.
No. You can however use PokéSav, a program which creates Pokémon with whatever stats you desire.
Because a Pokémon gaines EV on every stat, EV training just helps increasing the number of points gained for a single value. In short, a Pokémon will gain EVs even in stats it wasn't trained for.
Yes, you can do the "box trick" by depositing it and withdrawing it again. This corrects the EVs that were just obtained from EV training