Engineer: Zero Sum

 is a challenge in Risk of Rain 2. It is completed as by finishing charging the Teleporter with zero Monsters remaining on the stage.

Tips

 * The easy method is to charge the Teleporter to 99% without killing the Teleporter Boss, at which point no more monsters will naturally spawn. Kill the remaining monsters then kill the Teleporter Boss to complete the challenge.
 * Can be done on Prismatic Trials.