Before I tell you how to do the Lv 100 Mew trick, you have to know how to do the mew trick first (ONLY get as far as right before you battle second trainer on the first trick, or you'll screw up and only have a Mew at Level 7);
Requirements;
~Have a Pokemon with the move Fly
~Cannot defeat the gambler in Route 8 (The route west of Lavendar. I'll just call it Route
~Cannot defeat the trainer with the Slowbro in Cerulean Cape
~Pokeballs (If you don't have a Master Ball, use Ultra Balls)
Exit the underground passage that goes to Route 8. When outside, without taking a step, save the game. You may mess this up, so you might be resetting the game a bit. Now, walk one step down and press start. If done correctly, you should have the start menu up. Press Pokemon on the menu and have a Pokemon Fly to Cerulean city. If done correctly, you should fly away with the gambler having a "!" over his head.
Now walk north to Cerulean Cape. You may have noticed your start button won't work. This is normal and is only temporary. Now battle the trainer with the slowbro and defeat him. Make sure that the trainer walks up to you or the game freezes.
Now, Here's how you get that Mew to LV 100 easily.
Perform the Mew Trick cheat, and make sure you meet all the requirements from the above cheat. You must also have a Pokemon with Growl.
After doing everything of the cheat before this, when you battle the guy with a Slowbro, you must use the Pokemon with growl. You have to use Growl six times. Exactly 6, or the cheat wont work.
After using Growl six times, kill Slowbro and go to Route 8. The start menu will pop up, you press B and you battle Mew. BUT, there is something different with the Mew. That's right, the Mew will be at Level One. Yes, level ONE.
Catch it. Now make sure Mew gets EXP that won't let it level up. So let it battle a wild Caterpie or something. I'd advise saving the game (The start menu works after battling the second trainer) before doing so, because you might screw up. If it gets an amount of EXP less than it needs to evolve, the game will think of it as a negative level and make it 256. Now, the game will get confused by this and set it to level 100, an appropriate level. You now have a level 100 mew.
Why did it appear at Level one you may ask? Well, remember when I said to use Growl six times? WEll, I dont know much abut this, but it appears that the special of the pokemon you fought (Slowbro) creates the Pokemon, in this case, 21 is Mew. The attack ranges from 1-13, with seven in the middle, making the level seven. If you use growl on the Pokemon, the level will go lower and lower until you hit no level. So 7-1=6, so use Growl six times and the level is 1. Level one didn't exist until Gen 4, so if you give the pokemon less EXP than what it needs, it'll go to Level 100. Give it enough to level up, and it'll be a measly level two.
Enjoy your Level 100 mew!