To beat ur rivals piplup with ur chimchar on the first battle, do the following:

Step 1)When ur chimchar has full HP give it a potion.

Step 2)Use ember on the piplup.After the flame hits piplup and before his turn press A,X,LEFT,UP,RIGHT,Y,Y,B. This should burn piplup.

Step 3) use leer until the burn affects piplup twice. after it affects piplup twice use scratch twice then ur ember again. Piplup should have 5 HP.

Step 4) this last step is amazing!!!! If piplup has 5 hp and u have done every thing correctly this should work. Use leer. after that burn should affect piplup leaving him with 3 hp. use leer again. If u did every thing correctly leer should kill piplup!!!!!!!!! Its AMAZING!!!