The Spy Oculats that spawn during Death to Spies can do so during any battle during an adventure, including those against weak monsters. The trick is to draw an individual fight out until an Oculat spawns, then kill it immediately.
Kill on sight.
Let’s say you start a fight against one defensive and one offensive enemy and you’ve got the Death to Spies objective. Kill the offensive enemy, then kite and survive the defensive one until the Oculat spawns. Quickly tab target to the Oculat, kill it, then finish the fight.
Multiple Spy Oculats will not spawn in a single fight, but it’s easy to draw out just about any fight until one spawns. If you do not kill them in time, they will disappear – so be sure to kill Oculats immediately.
With the above in mind, you shouldn’t have any issue completing Death to Spies every time it comes up. Happy hunting!