Yep, this is a new rule, but I'm doing better with it now. I just look for people about 20 or so bigger than me with no defense items and attack them, and I'm winning 87.5% of my attacks so far.
Here's a tip, though: Focus on buying defense items. Attack items will just further restrict who you have available to attack, as you can only attack someone whose overall defense bonus lies somewhere close to your total offense.
I'm not advising we give up on attack items altogether, but with the new rules I think it makes sense to focus more on bolstering your defense first...