As Nath, use three battle cards in battles to gain a stack of 3 Extensions and then KO three other players in one life.
You can fight smaller enemies in order to build Extensions with Nath, as they are not as threatening as a boss or another player. Cards such as Rainbow-Colored Circle and I'm on Fire! should make gaining Extensions easier as they are low in cost and level requirement. To assist in getting the 3 KOs necessary, cards such as Sealed Guardian, Little War, and Accel Hyper can help Nath KO enemy players.