The player fights Mephista and the other puppet, Old Nick, at first. Once they are each killed twice, the bookcase explodes to reveal Mungola. The puppets will continue to respawn, attacking the player as they move about. Each time they respawn, they do so with higher HP. Mephista can heal Mungola, but not very much.
Old Nick and Mephista will respawn endlessly, and each give 1,000 experience and 250 gold when killed, substantially more than any other enemy outside of bosses. If the player is able to defeat them simply, they may wish to consider drawing out the fight, fighting Old Nick and Mephista over and over to level up and gather money. Weakening Mungola first is advised, so when the player tires of the battle and/or runs out of healing, they can end the fight quickly.
Eventually, Old Nick and Mephista's HP will increase to several thousand, at which point fighting them is still profitable, but increasingly impractical due to the time needed to slay them.