To battle with Lou using Easy Mode notes, try the following. When the battle with Lou begins (on Expert Mode), strum without holding any frets down. When you lose, select the Tutorial option at the Game Over menu. Do the Guitar Battle Tutorial and play through it in its entirety. After the "Lesson Complete" message appears on-screen and the next fret board appears, pause the game and exit. This will bring you back to the Lessons menu.

From here, press the red button, which will bring you back to the list of songs. The battle with Lou should be highlighted. Choose the song, and Lou's note chart will be set to Easy Mode now. Your note chart will still be in Expert Mode, but Lou's will be in easy mode.