Only rentals? I dunno, it might be very difficult without chance. Well, to get started, Zapdos and Articuno are a must. Zapdos is probably the best mixed attacker and Thunderbolt destroys most everything. Exeggutor destroys most of Bruno even without the optimal Psychic; Mega Drain is an OHKO for all rock/ground types and Leech Seed + Hypnosis can prove useful, but both those attacks are inaccurate.
For Lance, the biggest problem is Charizard, not the dragons. He will probably use Fire Spin tactics. Something that resists Fire Spin and sweeps Charizard? Omastar / Kabutops. Both are 4x resistant to Fire Spin, 2x resistant to Slash, and carry Hydro Pump. They are also resistant to Ice Beam, which is SOMEWHERE on Lance's team (and in Rival's Gyarardos on the non-rental team). Once Charizard is gone, you can safely switch in Articuno and ICE BEAM everything else. I also had a rental Lapras on my team but never used it.
Mime can prove useful as he's probably got the only good Psychic moveset. I loathe the level 50 rental sets. They're horrific. I mean, I couldn't beat Blaine's gym with them (brought in Amnesia Mewtwo for the lulz). --- ~Kurockbo~ "I played this game and beat it in three games."- darkmega12
The rental pokemon are always messed up. But a good tip is you should pick a pokemon that has the attack "Toxic." It does more damage every round. It's different from poison attacks because they just do a set amount of damage per round. I found that out the hard way when I kept losing my Mewtwo due to the fact it was doing like 70+ damage per round. --- Whatever you do, don't tell the good guys your plan. James Bond will come and ruin it.
I forgot, I remember I used only 5 pokemon though, and used a random pokemon like caterpie to do nothing, just so I can get the uselss pokemon into the hall of fame (I didn't own the gb games at the time).
I only remember that Jolteon was in my team. It was perfect against lorlei.