Building Routines
Source GM Core pg. 114
A complex hazard has a routine each round, whether it stems from preprogrammed instructions built into a trap, instincts and residual emotions swirling around a complex haunt, or a natural phenomenon like mutation fog. Build a routine that makes sense for the hazard; a chute that ejects lava into the area each round shouldn't be able to precisely target only the PCs, but it might spatter random areas within range or everything within range, depending on how you describe the hazard. However, a complex haunt might be able to recognize and target only living creatures.
If you create a hazard that can't consistently attack the PCs (such as an electric fence, which only damages creatures touching the fence), it can be deadlier than normal in other ways.
The hazard should have as many actions as you feel it needs to perform its routine. If you split the routine out into several actions, you can also remove some of the hazard's actions once partial progress is made in disabling or destroying it; this can give the PCs a feeling of progress, and it can encourage them to handle the hazard if it appears in an encounter alongside creatures.
If you create a hazard that can't consistently attack the PCs (such as an electric fence, which only damages creatures touching the fence), it can be deadlier than normal in other ways.
The hazard should have as many actions as you feel it needs to perform its routine. If you split the routine out into several actions, you can also remove some of the hazard's actions once partial progress is made in disabling or destroying it; this can give the PCs a feeling of progress, and it can encourage them to handle the hazard if it appears in an encounter alongside creatures.