When your level is no higher than 36, only a victorious NPC battle will cost you 1 MCU (excluding the battles against the NPC Legions, e.g. Donjerd Legion, Jackluis Legion and so on). From level 36, whether you win or lose, each NPC battle costs you 1 MCU. MCUs replenish 1 every hour. When you upgrade your Town Center (TC) you will receive an additional MCU. MCUs can also be purchased using Gold.
MCU Cool Down, akin to Builders Cool Down, will increase its duration as you fight more battles. Durations of all Cool Downs reset at 05.00 AM every day.