What pokemon do the NPCs use when allied for multi-battles

The first step would be acquiring a Mienfoo, which can be found in Route 14, Dragonspiral Tower, and Victory Road. The easiest way to breed for a specific nature is to find a Ditto with the nature you want the offspring to possess. From there, give the Ditto an Everstone and put the Mienfoo and the Ditto in the Day Care. The Everstone makes it so that the offspring has a 50% chance of inheriting the female parent's (or Ditto's) nature.

After this, you would just breed over and over until you get an offspring with 31 in either Attack or Speed. Give that baby Mienfoo a Power Bracer if the 31 is in Attack, or a Power Anklet if the 31 is in Speed (this forces that IV to be passed on). Repeat the process until you have a 31 in Attack and Speed, and hope that the ability is Regenerator.

_{I forget if you need to have a different parent in order to breed the offspring, or if you can just use the same Ditto again.}

Checking IVs

The easiest way to check for IVs is to stockpile a lot of Rare Candies. Take note of the Pokémon's stats at each level, and input them into this calculator. The more levels you take note of, the more the calculator will be able to pinpoint your IV. (You can reset your game after testing each Pokémon so you can use the same Rare Candies over and over.)

Their was originally a 50 pokemon limit in the NA version, but it has since been removed:

At some point in 2010, the limit that meant that players could only ever send 50 Pokémon from the Friend Board was removed.