The Zero Sum challenge for the Engineer in Risk of Rain 2 wants you to kill all of the enemies on a stage before the teleporter charges. Most players may not be able to do this reasonably, but there’s a standard, easy-to-follow trick you can to earn this achievement and complete the challenge.
You want to play through the stage as you usually would, killing a few enemies, purchasing chests, and then finding the teleporter. When you find the teleporter and the boss spawns, you want to stand in the teleporter’s field until it reaches 99 percent. You can damage the boss that spawns for a little bit, but you don’t want to take out the boss, which is a huge factor. Keep the boss alive until the teleporter reaches 99 percent, and then leave its field.
From there, you can freely wander around the map to start taking out and kill the rest of the smaller minions scattered across the map. With the boss stuck by the teleporter, and the charge nearly finished, no new minions should spawn in on your stage. After you kill them all, return to the teleporter, take out the boss, and when the teleporter’s charge is complete, you should receive the achievement and complete the Zero Sum challenge.
These challenges were introduced in the new Artifact update for Risk of Rain 2. We have all of those artifact codes mapped out for you, and detail the process you need to go through to unlock them. There are 16 artifacts in total, so good luck working your way through the game and unlocking those, along with the other survivor challenges.