You can gain all 3 season achievements and not have to do the harder difficulties.
First complete the season mode by winning/defending title at wrestlemania and save straight after.
Then continue as normal until you see the credits and the achievement unlocked sign should show.
Restart the Xbox 360 while on the credits and go back into season mode, change the difficulty to something different.
View the credits again, and another achievement should be unlocked.
Repeat this for the other difficulty and get the achievement for that too.