How much difficult it is for player, it reaches from 75lvl to 78? Where will he be supposed to xp-ing? What mobs it kills? And finally how much time it wants?
It's something like 200mil XP to go lvl76-77, and that can take a long time. 77-78 is like 300mil XP, and 78-100% like 400mil XP. I really don't know if those numbers are correct but it certainly seems like it.
From what I understand, mages should be in Hot Springs and melee should be in the Imperial Tomb or Forge of the Gods.
Either way, you should go to the C4 areas - Forge, Imperial Tomb, Wall of Argos. Varka Silenos/Ketra Orc places are good for groups.
As for time - I have no idea. I know people who have grinded (ground?) 76-78 already since C4 began - that's 3 levels in about 4ish months, and these people play in any party they can get.
In addition to what my clanmate says above, I'd suggest anyone 76+ to focus on the new areas for XP. The mobs are much harder, yes, but they also give the best XP.
I friend in another clan put his BD in ANY XP group he could get, regardless of where they were, because at 76+ you actually don't care where the XP comes from as long as you get it.
We're making our way deeper into the Ketra Orc Barracks, killing Generals and Commanders, mostly. If you get the Alliance quest from the orcs or silenos, it makes grinding here a bit worthwhile (You are increasing your alliance level as you collect quest badges). We're doing just fine in a party of two, with out of party buffers.
I've been in parties in Imperial Tomb but with the fast spawn rates, you don't want to do this place solo but you don't need a 9-person party either.
So at the highest level, just get XP where you can. It took about 2 months for a friend to go from 77 to 78 -- and he played a lot.
So its soooooo difficult hu? To many months of continues play :-( It scare's me...
Don't be scared. :) The XP/SP needs for 76-78 are so completely insane that you truly won't pay attention to them at all. What I mean is, you know how from say 70-75, your whole world seems focused on getting to 75, doing the various quests, and then subclassing?
Well 75-76, you just want your 3rd class quest done. Most melee classes have such crappy skills (Wisdom/Health) to look forward to that they mentally skip it to focus on the next level.
At 76-77, the SP requirements are laughable. But most classes have at least one decent skill that they can get excited about. So you'll grind knowing eventually it will be a good reward. Plus you realize you may need to farm some SP, so this grind time can be viewed as a good thing.
From 77-78, you realize (at least if done before C5), that you're reaching the "end game." You'll be the highest possible level in the game, but the game will make you pay for it. :) If you have enough SP, you can buy the best possible skills created for your class (at least in NCsoft's eyes).
If you have the time, you may as well go 78 and 100% since in C5 the level cap moves to 80.