Well i didn't played it at the hardest difficulty so i can only try (it's been a while ...) to recall.
1) usual way mob farming? when i played it i was just farming levels and money at the same time
2) you don't. when reaching level 3 you lose virginity if you haven't already
3) there are items that raise your max cap that you get thoughout the game. sleeping is the best way since the progress of the story is time based and you may find yourself with nothing to do but pass the time to progress (not sure if in hardest dif. it holds true), there is an item which is limited that is tied to the number of star pieces that a person in the noble district gives you that raises energy, however i would only use that item for one-day finish some long quests. That and other than bath nothing else and from what i remember you can't bath more than once per day
