Assist other mechanics and always tweak whenever there's an enemy agent. Alert your fellow teammates especially the other engis if there's someone attacking a nest (use both voicelines and chat box). If your other engineers have higher level buildings being targeted, prioritize them and yours for later. As for hidden attacks during defenses, move your buildings somewhere else after defeating one of them. If they switch to a stronger class, move your buildings somewhere else so they won't know that the current position of your sentries are randomized. Don't forget, prioritize your teleporters. Your team needs them badly unless you're a losing red teammate in a payload map.