This is a much easier way to beat brock and get Mew at the same time!
First, put yello into your xploder cheat cart. Then when it asks you what cheats you want to activat, select the one that says something like 'catch mew'. Now go up to brock with the cheat on and when you talk to him he will act normal until the battle starts. Instead of being brock, it will be a wild mew. If you catch it or kill it, the battle will be over, and you will get the boulder badge!