sectors 3 (green), 6 (blue), 9 (purple) , 12 (orange drop) are just like other sectors but on hard mode. you go with 5 other players and complete an allod by defeating 4 bosses. Theratically u can use mercenaries but u'll not get far with them, so don't even try that. U need real players that know what to do with reasonable dps.
Btw, since my apprentice had problems to go to those sectors also, i'll mention that u cant go from que menu (dunjeons tab) to those 4 sectors, u'll need to tp from ship by using the astralblade (its an option on the map when u r on ship).