Learn about Effort Values, then come back later.
Not interested in learning the details? Ok, short version:
RARE CANDY DOES NOT WEAKEN YOUR POKE ONE IOTA.
Longer version:
A lack of Effort Values will weaken them. EVs enhance the stats of your Pokemon and are primarily acquired through causing enemy pokes to faint (the type and quantity of EV gained is based on the species defeated).
Stats get recalculated each time you level up, which includes the current EV modifier, so even if you leveled up to 99 on candy, went out and brought your EVs up to snuff, and then used one more candy, you would end up with a pokemon that is identical in power to one raised through grind.
If you're into the competitive realm, it's common to just skip the grinding bullshit and just edit your pokes as necessary (within their legal limits of course. Duels with hacked uberstats/skills/typing lose all significance and don't prove your ability as a player one bit. Plus, it's boring). It's the one instance where I can approve of using a PAR or similar cheating device, because unless you really want to waste weeks of your life grinding one team (and breeding egg moves...ugh) it isn't necessary.
In terms of competition: Every argument I've seen for doing it the "legit" way is sentimental bullshit. Don't try to justify wasting time doing absolutely nothing; It does not make you a better player.
The only argument that works is that neither player has access to a PAR (since you can easily share one between games), in which case it becomes a non-issue anyway.
The single player "story/campaign" is in every pokemon title I've played is balanced to not require power-leveling to win (as long as you know how to match types or where to find certain key Poke's), so it shouldn't be necessary there.
...Did I seriously just write two and half paragraphs on fucking pokemon? Damn, the boredom is really setting in thick today.