Begin a Season mode in any difficulty and play through it like normal. After the Unforgiven PPV, save your game and proceed to next week. The credits will then roll and you will get an achievement for season completion. During the credits, return to your Xbox Dashboard by pressing the Guide button and selecting "dashboard". This lets you reset and start the game over after earning your achievement. Go to season mode and choose continue. Before proceeding to next week, change the difficulty setting to something else and then select continue week. You will then get another achievement. Do this one more time for the last seasonal achievement.