How to Get Level 100 Pokémon in Pokémon Red

Steps

Teach your Pokémon powerful moves with TMs if its moves are not desirable. This will make training easier. Make sure that the moves enough PP (aim for 25+ among all offence moves), at least while you train.

Battle Pokémon that your Pokémon is super effective against. This will make winning battles faster. Aim for Pokémon that are slightly higher levels than your Pokémon.

Use an Exp. All and use a more powerful Pokémon to battle even stronger Pokémon. Keep only the battling Pokémon and the one you are training on your team, so as to not waste exp. points on Pokémon that don't need training.

Re-battle the Pokémon League when your Pokémon gets powerful enough to. This will gives lots of experience. For a change of pace, you may want to try training in Cerulean Cave (a.k.a. the 'Unknown Dungeon').

Rare Candy Glitch Method

Place your Rare Candies in the sixth slot of your items. It's important that you place it in the sixth slot, because otherwise the glitch won't work.

Place Pokémon in your party that know Fly and Surf.

Talk to the Old Man at the north of Viridian City and ask him to show you how to catch Pokémon.

Fly to Cinnabar Island. You must fly, if you walk or surf there, the glitch will not work.

Surf directly on the east coast of Cinnabar Island. You must surf at only one 'square row' east from the land.

Run from MissingNo. when it appears and you will then have 128 Rare Candies, regardless of how many you had originally.

MissingNo. Glitch Method

Place Pokémon in your party that know Fly and Surf.

Talk to the Old Man at the north of Viridian City and ask him to show you how to catch Pokémon.

Fly to Cinnabar Island. You must fly, if you walk or surf there, the glitch will not work.

Surf directly on the east coast of Cinnabar Island. You must surf at only one 'square row' east from the land.

Catch a Pokémon over level 100 when it appears. Make sure it is a real Pokémon and not a glitch Pokémon, which may cause unwanted side effects.

Make the Pokémon win a battle and it will revert to level 100.