alright, heres what you do:
1: attack the previous poster with something. make sure it can LOGICALLY break the shield of the previous poster. for example: a shield of light can't be defeated with a holy attack.
2: defend yourself with some type of shield that isn't indestructible. it has to be able to be destroyed by SOMETHING.
rinse and repeat.
only rule: make sure your shield can be defeated, and make sure your attack can logically break the previous poster's shield.
1: I defend myself with a shield of light!
2: I attack with a beam of darkness! I then defend myself with a shield of paper.
I defend myself with a shield of light!