Is there a fast way to get Pokemon to lv.100?

Nope unless you use an action replay or gameshark

Well you can go to the day care centre, or use an action replay.

Only with Rare Candies or a Gameshark or Action Replay code.

Yeah, here it is.
You need
Pokemon (duhh)
One of those things you plug in the gameboy and then plug in a socket to charge your gameboy up
OR
Batteries that can run for about 10-12 hrs without running out.
Something heavy (I used a full 750cl vittel bottle of water, you could find something else though)
1. Bung 2 Pokemon you want to go up levels into daycare (Note: Lower level Pokemon go up more levels so don't bother training it.
2. Go to fallarbor town and head right so you go through the place with brown grass. When you come out the other side, go to the little berry patch where there is a girl who gives you a razz berry. GO down a bit then into the desert and you will see a little slope which you can't get up unless you have mach bike. Do not go and get the mach bike!
3. Get your heavy object. Stand your character in front of the little slope, then put the heavy object onto your gameboy's forward key so your character goes up and down the slope.
4. Plug in your charger/swap batteries for longlife ones and leave overnight.
5. Come back in the morning, your Pokemon will have grown! (If it's not the right level yet leave it in daycare unless you really need it). It won't grow 100 lvls overnight but if you do this over a period of nights then you will eventually get a lvl 100 with no training! Oh by the way you will need Heart Scales so that you can learn moves that you had before. If you can't find any just go to Ever Grande city and fish at the waterfall and catch Luvdisc. Some hold heart scales.