[popsicle_3000]

A few comments:
-I dont like Oni as a term. most people would have no idea what that means (including me)...
-I've always been puzzled by having easiest AND beginner as different difficulties. to me they mean the same thing... thanks Middie for your suggestion.
-I like for legends only. makes FLO. not the greatest acronym, but better than FSO looks anyways.
-bahamuts suggestion "Impossible" i find fitting. nothing can be harder than the impossible ;) I know... bit sappy
-I'm against flashy lingo. I recon difficulty names is something that should be pretty low key.
-I guess i'm partial to "Very" since they've been in the game already...

I've combined my favorites. I'm not a huge fan of my 9 & 10 suggestion, but i'm not entirely sure what else would be good...

01- Beginner
02- Very Easy
03- Easy
04- Light
05- Moderate
06- Standard
07- Tricky
08-Very Tricky
09- Heavy?
10- Very Heavy?
---
11- Difficult
12- Very Difficult
13- Challenging
14- Very Challenging
15- For Experts only
16- For Masters only
17- For Gurus only
18-For Legends only
19- Ridiculous
20- Impossible