enemys have about double the armour durability and their damage is more them doubled from crusader difficulty, however your VP from your last playthrough carry over so you can fully develop all your stats by about halfway through chapter 2 but that won't stop two handed weapons taking over half a full tier of health with one hit.
The first time i had issues was chapter 3-1 when you're on your own and get swarmed by at least 15 enemys, running through to the end and backtracking worked eventually for me but it wasn't easy.
boss fights are normally an issue due to the infinite spawns of enemy, but if you manage to corner the boss, it's not really an issue ... unless your weapon breaks.
the final part of the game i wasn't looking forward to but i did it first time by constantly spamming fire maelstrom (even if i did get downed a fair few times)
all in all, aside from the occasional brutal part (chapter 5-1 for instance) it's relatively easy if you don't give up easily - good luck :)
*Edit 1* - Chapter 3-3 i had issues with on crusader difficulty at the end when you're seperated, however the level ends when you hit a certain area on the castle wall so running through and swearing at the camera angle on nightmare should work out if you're skilled with deflection counters (which shouldn't be too much trouble)
*Edit 2* Chapter 4-3 has a familiar boss at the end of it, however, this time he's a nightmare - on the first part i stayed back and attacked him and his forever spawning 'goons' at distance with the crossbow to take some of his armour off, then i rushed him and attacked in curse mode until part 2 started, then i got extremely lucky and managed to corner him where button mashing X killed him relatively quickly