Lucifer is the hardest boss in the game, if i recall correctly he has something like 999,999 HP or greater, he has a few lethal moves for the unprepared but he isn't impossible. Best bet for the fight is to take a Metatron demon with you, and then use Dante to full effect. A word about Dante though once you have him in your party you can't get rid of him and you can't swap him out so bare that in mind
I think that as long as you agree to undertake the labyrinth (and the fiend hunting) then it remains open, not matter in game decisions to make, i mean if you wanted you could get Dante on level 5 of the labyrinth and then leave and still get any of the other endings.
Normally i try to level up my character evenly, but i think it all depends on how you normally play, it takes a while to get a few decent magic attacks while you start with physical attacks. Also its well worth hunting down the final magatama, i can't remember where it is though but i don't think its part of the main quest but there's some good stuff in there.
Well anyway i'm going to call it night here, i'm up early in the morning (for a change) so we'll resume this later
especially since i now have a real urge to play this again, must get it back off my friend. Talk to you later :thumb: