I'm going through this game at the moment with rentals, as a bit of a challenge and it is very hard!! Rental pokemon differ from stadium to gym leader castle, the best rental pokemon have at least one attack which matches their element, complimentary moves which aren't repetitive, high accuracy moves (since lets face it the computer cheats), ones which have combo moves and also different types of moves, like more than one element. Make sure their moves suit their stats, don't go for a pokemon with lots of physical attacks and no attack stat to use them. I have done all the way up to the rival in gym leader castle round 1 with rental pokemon so far, the little cup, the challenge cup lol they pick them for you, the poke or nintendo cup up to master ball and will head to the prime cup next. Let me know what your up to (if your still doing it) and i can tell you what i used to get through it. I just finished poke cup ultra ball division using gengar, jolteon, moltres, omastar, onix and skiploom. Skiploom funnily enough saved me the most.
