A bunch of guard with lousy perception skills made our party almost walk up to them before some of the slaves cried out. This was made to be a walk over and it served that. The party called out the poor soon to be dead Aspis folk, and without too much party in good old pathfinder fashion, the guards was killed off.
How to place them in the camp site is pretty much up to the GM except for the large wooden structure. Those made me think of different ways after I had played the scenario. Some ideas for flavor could be: They come at night (or dusk) providing a rare chance for rogues to use their stealth. The guard’s placement could be around the camp maybe even standing guard in front of a tent where Zahur Karn would rest.
I had them all patrolling around the wooden structure and lucky me we did not have a fireball wielding player otherwise everyone had been dead to begin with (they almost were anyway but that’s beside the point)
Spent some time planning how the camp should look maybe make small paper tents on the flip map and make it look very important.
My experienced was the party of experienced pathfinder players kept count of the encounter (Encounter 1 Erinyes Check, Encounter 2 Guards Check)
But the real encounter is inside the wooden building, with guards having heard the battle and being ready. Yes they too are no match but again the surprise for the player walking into the building that "encounter 2" was just beginning was priceless.
This part worked well because of the surprise otherwise it’s just another dull "I remove the guards" encounter. But as mentioned it does not have to be like that. The GM has free hands in the descriptions to make it a cool looking battle scene.