to go to sos you need Scroll of Seekers aka (SOS)!
1 sos grants you 88mins in there!
to go there you need to go to guardian captain in your home town and talk to him!
i dont know exactly what lvl you can enter there but from m1 - m33 there are 4 sos maps!
infernal island, cardinal island, maruda island, ancient island !
from g1+ there is 3 sos maps called Sky temple exterior 1, 2 and 3!
in sos is exp boost x7 all the time!
hope it hepls!