Game will pretty much start off very rough but at least doable if you're decent at Castlevania titles, may take a few tries for each stage though (also really depends on character you pick too, Soma or Alucard will have a pretty easy time right off the bat due to the way they work, other characters not so much). Once you finish Normal mode, Hard mode will just slaughter you and you'll wonder what the fuck to do next. At that point it largely depends on your character again, a couple of rare drops can turn Soma or Alucard in to a God, whereas the other characters have to grind up their skill levels to power up. You'll eventually reach a point where you just destroy anything in the base stages regardless of character. The DLC is all basically progression after that, in the 360 version it was months before we got them all and once they were released the old stages were pretty much abandoned, almost no reason to do them since the DLC stages are both much harder and give much better loot. You basically just move through them until you're capable to beat the next stage in each (though chapter 10 is harder than 11 imo). Depending on your character you'll want to grind specific stages for certain things. The female characters all get their best body piece from Chapter 7, Soma gets some ridiculously good souls from 8 and 9 and broken weapons from 7 and 11 (6 too but the drop rate is asssss), etc etc.
As for group vs solo play, it really depends how well off everyones gear is. The enemies all scale to the number of players, so if you're significantly stronger than someone and you run with them, the boss is likely gonna die slower, not faster. There's generally a few shortcuts through stages you can take if you have another person though, so it does speed up in that regard.
XI Wiki


