as dreamlab said, but have the a rule to say when the attribute is 5, increase difficulty, then reset attribute to 0. that way it can build up to 5 again and increase difficulty again.
If you don't already have an attribute for what round you are on, create it. Then, put in a rule that checks when that attribute is = 5. Create another attribute, call it difficulty. Then, in your rule where you checked to see if the round was = 5, put a change attribute behavior that will set the difficulty to difficulty+1 (use the function option to select your attribute and add the +1). Also, in that rule, put another change attribute behavior to set the round back to 0 so that you can start over and add another difficulty level when it reaches 5 again by using this rule.
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DL