elitefour_protector said:
Also a Trode with Explosion and T/Wave helps.
Only if you're desperate. After all, correct me if I'm wrong, but in the RBY series, aren't you barred from using items during a player-to-player battle? (It's been a long time since I've battled using games from that series.) If so, then you're pretty much screwed if you use Explosion, especially if there's not really a guarantee that the strategy will OHKO the
entire team. (Mew's a most likely. It's the Mewtwo you have to worry about.)
That said, if someone seriously loaded a team full of Mewtwo (and Mew), then chances are you could probably get away with just blasting the suckers with a high-powered move (e.g. Earthquake). After all, no offense to your opponent, but five Mewtwo and a Mew make me think someone's a bit lacking in the experience department, so you won't necessarily have to worry about strategy (unless I'm underestimating him or her).
Anyways, if I recall correctly, Mewtwo's main weakness is its Defense. It's still a bit on the stellar side, but it's still an opening that I highly recommend you use to your advantage.
Really, I'll have to agree in saying if you've got a Snorlax, you're pretty much set to go. You might benefit from a nice Chansey as well. (Light Screen and Softboiled alone will drive your opponent freaking nuts. Add in Bide, and you've got a Chansey that can actually take a few hits and deal some damage too. Granted, Chansey's not much of an attacker, but let's face it. Five Mewtwo at once means the basic rules are pretty much null. Not to mention it doesn't matter with Bide anyways.)