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We didn't really notice difference between level 20-40 and 60-80-100 boss. All in all hydra is annoying, the others are jokes since we would always take fanatic+Body Boost+spydrink+TP regen (most useful being fanatic for higher level floors' boss since it anihilates their special move).
The difficulty seems to scale to the number of persons, you often get only 5 mobs to kill or less in kill all floors, # lamps is usually 3or 4 (but it can, rarely, be 5). Joke floors are lamps and single enemy, annoying floors are when you get serket and other crap NM or "do not aggro gears" floors.
You typically reach the last floor with 10 min left. And to those saying "you can be boned" with bad floors, remember that with 3 you kinda double your chance at drops.
Non boss floors are joke with 3. We use THF+MNK+RDM for boss but you can use MNKx2 RDM for those. Also we do dual box so it's not as efficient as 3 real players (thinking specifically of sneak casting, curing on boss etc).