Everyone seems to be going to that Gambler in Fuchsia, then going back to the Slowpoke trainer. Well, it isn't necessary. What you can do which will save you a lot of time is right when you eneter Cerulean, don't battle the gym or its trainer, but go up Nugget Bridge. Beat all of the trainers on the path to Bill except for the youngster with the Slowpoke. To the left of Nugget Bridge is a patch of grass with a trainer, accessible once you have beat Nugget Bridge. Get an Abra, and do the exact same thing you do with the Gambler near Fuschia with him, using Teleport instead of Fly. The trick is that itdoesn't really matter what trainer you fly/dig/teleport away from, but what trainer you battle next. Now, there are two trainers in Cerulean who you may battle. Since you haven't gone all the way to Fuschia, you can battle the Youngster, and then go to Nugget Bridge. You will encounter a Lv. 7 Mew, as before. For a second
Mew, activte the glitch again by teleporting away, then go to the gym and fight the swimmer. Beat him, go to Nugget Bridge again, and another Lv. 7 Mew
is yours for the taking. It should be known that if you activate the glitch and battle another trainer, once you go back to the place where you activated the glitch (near Fuschia or Nugget Bridge) another pokemon will appear other than Mew. I am not sure how you can tell which pokemon appears when, but theoretically you can use it to catch growlith in Blue, for example. I got a Lv. 7 Mankey once.
Thanks to darth_schroodle and his wonderful guide for notifying me on this.