Play the China-town level after getting the Katar. Climb up the Chinese house that has the huge flame on the porch. When you are up there, someone will surprise attack. Kill him and go to the opposite side of the part you climbed up to. Go to the back of the house, but stay on the balcony. Jump to the building on the far side (all the way down, as far as you can see) and look around for another piece of paper. Walk into it, and you will unlock the 3 minute battle mode option.