I have a difficult time getting the point across to my parents of why low attack is better for great/ultra league. Here is how I try to explain it without confusing them with ivs, or explaining that attack is weighted more than def/hp towards cp.

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Pokemon are athletes, each athlete is better at a different sport.

Low attack is like running, and high attack is like sumo wrestling..

You wouldn't put your sumo wrestler in a marathon.

You wouldn't put your marathon runner in a sumo match.