*taken from the thread above
Bronzong is a little tricky to EV for, but there is one thing you should always do: maximize HP. Then, if it’s mostly an attacking set (such as the Trick Room or Calm Mind sets), put EVs mostly in the attack stat you’ll use. If it’s a supporting wall, like the pseudopassing or Sleep Talk sets, pump defenses.
If you’re using any set involving Gyro Ball, make sure you try to get your Speed as low as possible by using a –Speed nature and preferably a 0 Speed IV. This will help ensure Gyro Ball deals the most damage.