If you don't have a partner this also works on Split Screen Co-op Campaign with a second controller, just put it on Casual Difficulty. You do not have to do this in any particular order if you have already unlocked the solo campaign chapters. You may participate in a chapter online that you have not yet unlocked in solo campaign if the host has unlocked it. However, even if you complete it on-line in Co-op, you will not unlock that chapter for your solo campaign. You can check your progress on the level select menu with the metals - the Co-op badge at the bottom will be colored in if you have completed the level in Co-op.