Normally, leveling up your Pokemon to level 100 is very time consuming, but there are a few things you can do to make it MUCH quicker.
Things you will need:
Lucky Egg(s) (1-6)
A strong Pokemon (Lv. 80-100) (If you don't have a high level Pokemon, give one of the Pokemon you are training Toxic)
Get Lucky Eggs for each of the Pokemon you are training. (For example, if you are training 4 Pokemon you will need 4 Lucky Eggs) (More info on how to get Lucky Eggs here)
Scan the below QR codes in your Secret Base QR code scanner.
Other QR code
After you have scanned those QR codes, go to the secret bases that were put in your game with the QR codes. (It doesn't matter which one you choose.) When you get in the secret base turn on the Level Release. (This thing ----> ) (When it's on, the ;ights on it will start flashing.)
MAKE SURE YOU HAVE YOUR EXP. SHARE ON.
Activate Exp. O-Power. (Preferably level 3) (More info on how to get O-Powers here.)
Once you have turned on the level release, make sure your lv. 80-100 Pokemon is at the head of the party, if it is, talk to the trainer in the secret base and battle him/her. You will get battled by 3 lv. 100 Blisseys, the first 2 will use Healing Wish, when the third Blissey uses Healing Wish you will have to defeat it manually. If you don't have a strong Pokemon, use Toxic on the last Blissey.
Hope I helped!