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You're thinking of the Missingno glitch. It's not a person you fight, but rather a glitch Pokemon.
It also doesn't give you 99 of an item, but effectively adds 127 to the item in the 6th item slot IF it's currently got less than that amount. You won't get more if you've already glitched to get more.
It should also be noted that there's a known history of bad side effects with Missingno, the worst documented being my own cartridge being rendered unusable and several other people having their game saves corrupted. The risk of this is extremely small, however, so if you still wish to go on with it, by all means do so, but don't say I've not warned you in advance.
Now then, to do the glitch, the simple way, is very easy.
Go to Viridian City and head up, and speak to the old guy that shows you how to catch a Pokemon. Get him to show you, and then FLY to Cinnibar Island, then SURF on the right-hand coast, so you're half-on, and half-off the water.
If you want to manipulate this glitch, or learn the technical reasons behind it, the FAQs will give you a wealth of information on the subject.
Where logic won't work - I will PAL Gamers unite - http://www.gamefaqs.com/boards/277-pal-gaming