Instructions
Sign into a throwaway Xbox LIVE account on an Xbox controller. Sign into your own account on a second controller.
Load the game and play through the beginning part on the throwaway account. Press "Start" on the second controller when you reach the music box. Select the hard drive and wait for the game to acknowledge that your main account has joined in.
Press "A" on the first controller to obtain the music box to receive 50 Guild Seals.
Press "Start" on the second controller and save the game.
Press the guide button on the first controller and sign out.
Repeat Steps 1 through 5 until you have as many seals as you want. This will net you about 50 Guild Seals every minute.