Dynamic Encounters
No encounter needs to use all of the elements presented here, and not all encounters need more than one or two. The more complex a dynamic encounter is, the longer it takes to run and the more demanding it is. In general, these tools are perfectly suited for boss encounters, for memorable foes, and as a spice to add throughout your campaign however often works best for you and your players.
Hazards in Combat
As their name suggests, complex hazards are a more powerful tool for encounters. Because they continue to act, they're an ongoing presence in the fight. When combined with hostile creatures, complex hazards offer the PCs plenty of choices for what they want to do next. This is particularly true if foes benefit from the hazard. Should the PCs first disable the array of pipes spewing flammable gas into the room, or should they prioritize the fire elemental creeping closer and closer to the growing cloud of explosive fumes? There's no right answer, and the PCs' choices have a clear impact on the obstacles they face. Hazards in combat shine when they give the PCs ways to contribute meaningfully other than dealing damage to a creature. Interesting actions to disable a hazard are a fun way to give several PCs something fresh and different to do rather than piling on damage.
Evolving Battlefields
Similarly, a third party in the encounter, perhaps a rampaging monster or a passing security robot, could pose a danger to both sides but potentially benefit either. For instance, perhaps the PCs or their foes could harness this third party as a dangerous but powerful ally with a successful skill check of some kind or by making a risky bargain.
Sometimes the evolving battlefield is more of a state change, or series of state changes, and less of a constant presence. For instance, defeating the rogue AI controlling a space station could cause systems aboard the station to malfunction, perhaps turning off the artificial gravity and forcing the PCs to deal with the remaining robots while floating. Major physical changes to the environment, such as the collapsed portions of a room rising and falling or water beginning to rush in and fill the room, can force the PCs to rethink their plans to handle the new situation. Sometimes the evolving battlefield is more of an unexpected plot twist that occurs in the middle of the encounter. Perhaps the evil tyrant reveals that they were a dragon all along, or security robots arrive to help whichever side is outmatched. Whatever you choose, make sure it changes things up and makes the encounter feel more dynamic and different. For instance, raising up a portion of the battlefield that isn't particularly relevant when neither the PCs nor their foes are likely to care is less interesting than raising up the pedestal holding the datapad the PCs and their enemies are trying to recover.
Combining and Separating Encounters
The most common reason to separate an encounter into multiple pieces is to set up a combined encounter, like when an injured foe retreats to gather reinforcements. This provides the PCs with a choice: do they ignore the fleeing enemy and focus on the battle in front of them, or do they split their own forces, weighing the risk of being led into a dangerous encounter against the chance of stopping later foes from preparing for their approach? An encounter might also separate into pieces because of dramatic changes to the battlefield, such as a collapsing ceiling or a magical wall that prevents those on each side of the barrier from accessing the other without spending actions to bypass the obstruction.
Time Pressure
Secondary Objectives
Sometimes a secondary objective might present a time limit, like if the PCs need to prevent evidence from being deleted, either by fighting quickly or by disconnecting or stealing the computer it's stored on. Another type of secondary objective relates to how the PCs engage in combat with the primary opposition. The PCs might need to use nonlethal attacks against security guards who mistakenly believe the PCs are criminals, or they might need to prevent security drones from retreating to alert others. Options like these highlight mobile characters like the operative. You could even create truly off-the-wall secondary objectives that require the PCs to lose the encounter in order to succeed. The PCs might need to put up a believable fight but retreat and let foes steal their urban cruiser in order to follow the foes back to their base. Secondary objectives are a great way to highlight different abilities in combat and make for a memorable encounter, but—like all of these tactics—they can become annoying if overused.
Opponent Synergy
When taken to its extreme, synergy can represent the actions of a hive mind or a single massive creature. These synergistic components can be creatures, hazards, or both. For example, instead of representing a previously unseen Swarm entity the size of a warship as a single foe, you could represent each of its pincers as an individual opponent. Perhaps this bespoke Swarm creature can sacrifice actions it would otherwise use to crush PCs in its maw to use its pincers more freely. In this case, you could model a field of pincers as a complex hazard that mainly reacts to the PCs moving within it, but allow the Swarm's body to act with a few pincers directly.