Yeah, I used thrash repeatedly and got lucky xD
I faced him three times, all three times my Gyarados killed his Pikachu but the rat left me with red health.
Then Espeon would come in, use Psychic, kill Gyarados. From then on, all my Pokemon (Typhlosion, Ho Oh, Snorlax, Slowpoke, Scyther - all around level 50-55) would get murdered by Espeon haha.
So finally, I stopped for a bit, pulled out Mr Mime (around level 40 at the time) and trained the shoot outta him on Mt. Silver, eventually he got to level 72 and knew Psychic, Solar Beam, Fire Punch, and Zap Cannon.
Took out Pikachu after Gyarados thrashed him to death (this time it was easier, Gyarados was lvl. 60 at this point), weakened Espeon, KO'd Gyarados, Typhlosion (55) kills Espeon, Mr. Mime solar-beams the crap outta Blastoise, I used like three fire punches to take out Venasaur, I had to waste like my entire PP of Psychic on his annoying Snorlax (xD), and then finally, revived Gyarados and one Hydro Pump took Charizard out. ^^
and yup, totally agreed, Mr. Mime is awesome. Finally someone that agrees! :D