I was playing arenas all day and after few tests with a friend i saw that when you use Expose armor on a warlock, before he uses his demon form he won't get any bonus armor. Why ? He will be with 0 armor, and when he uses his demon form he will continue to be with 0 armor, not 18-20k. If you use expose armor AFTER he gets his demon form you won't benefit from this. I am not sure it is exactly bug or exploit but i am sure a lot of people don't know it. Just use expose armor in the beginning of the fight and the lock will be GG even with Demon Form =)