Halo

I've almost completed the map, cleared the entire upper world of enemies, and only the pink area remains in the lower world, but I can't seem to get to the lower world. I suspect there's a portal in the lower right corner of the map that leads there, but there's a random monster on the only unoccupied piece of land. The only way seems to be by flying, but that option isn't available. Or is there another way to get to the lower world?
P.S. The map is interesting.