Sorry, I can't get a screenshot at the moment myself, but I'm pretty sure he meant Asphodel, not Acheron. (I'm guessing this is your "grass-like" area.) The southern-most room has a large plant in it. Two rooms above that is where Asphodel's skull can be found. It's easy to miss because it's inaccessible when the vines are still in place, and once they're cleared, players tend to forget all about the room.
Elysium's is in the room directly above the maze path. Acheron's is in the above-water room right before the boss. Styx's is in the southern-most room with the ice blocks. I don't immediately remember the others, but if this doesn't help we can get you a full list in awhile. Sounds like your remaining areas are Lethe, Asphodel, and either Phlegethon or Jackland.