Where and How to improve the pokemon to get through Nimbassa City

Feed him 11 Pomeg (lowering HP), Kelpsy (lowering Attack EV), Qualot (lowering Defense), Hondew (lowering Special Attack), Grepa (lowering Special Defense), and Tamato Berries (lowering Speed).

If a Pokémon has over 100 EVs in a stat, the EV-lowering berry will reduce it to 100 no matter how high it is, and after that, each berry will decrease the stat by 10 EVs after that. Because of this, feeding a Pokémon 11 of a stat decreasing berry will always result in 0 EVs in that stat.

This is assuming you can gain access to the EV berries. As far as I am aware, you can't find them in game yet. These berries are the only way you can decrease EVs.

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.)