It's all I have left to do to get into the blue building but I'm not sure how to do it.

You can do it by training your pet in the Training centre in Riverside Valley. The rating means overall stats. I'm not sure if Levels count toward this achievement, but pretty sure Health, Strength, Defence and Speed do.

the overall rating of 100 just means one of your pets needs stats (strength, defense, health, and speed) that add up to at least 100 points (so 25 in each stat would do it). not really sure why it's worded in such a strange way, but that's all it means.

Ohhhh ok! Thank you so much!

Also thank you !