Copying this from Contrary's and Skuntank's blogs
You are going to design a gym, choose your specialty. (ex. dragon type), choose three teams to battle trainers of different skill level.
A team for a beginner trainer with no badges, choose two pokemon level 9-14.
A team for a decent trainer with three badges, choose three to four pokemon level 21-30.
Lastly, a team for a trainer challenging you for their eighth and final badge. Choose four