Well from personal experience, I'm aware this particular levequest has a horrible highlight issue for mapping the location of the cactuars. You need to kill 7/10 to have the final 3 cactuars spawn and it is very likely that the one you missed which was no where near any of the highlighted regions on the map. You may actually have to search as far as a entire map square distance away from the yellow circle spots to find the remaining cactuars. I recommend next time that you search far east from the normal spots and hope to see the red dots on your mini-map which is where they usually are. My first experience with this levequest with friends had us searching half of the region to find the remaining ones.