it says 'Have at least 1 pet with an overall rating of 100 or more ' does this mean battle stats??

Yes, It means Battle stats. Add your level, Health, Defense And Strength of i] One [/i] pet, together and the sum needs to add to 100 or over. Dont include your Intelligence.

Yes, that's right. Level + Strenght + Defense + Health = 100; Intelligence does not count on the other hand. C:

By training your pet in the Riverside Valley

[edit] You can spend exp on stats in the battle colliseum. You can use the Gym Vending machine too