Hmm i dunno, i guess the enemy combinations can be a greater problem than the length itself, in which case lightning chariot and final battle could be hell since their lengths allow for more waves of enemies, but it's been a while since i played those and i don't remember the waves of enemies very well, i think Final Battle may not be very hard for i remember my only death there was getting eaten by Tempura Wizards a fight that can be avoided and touching the... sigh... Orne...
Did all the chapters using a Value 308 Rose Staff with 4 Ranged and 2.5 Melee
Overall Defense +2 / Health +3 / Dash Ch. Shot +3 / Standing Cont. Fire +4 / Status Resistance -1 (This status -1 really screwed me over against Phosphora)
For the Chaos Vortex though i had to bring something with better defense 'cause that really is an endurance contest, in this case a Value 298 Dark Pit Staff with 2.5 Ranged and 0.5 Melee
Overall Defense +7 / Shot Range +3 / Side-Dash Cont. Fire +4
For powers, Double Item lv.3 / Crisis Recovery lv.1 / Health Recovery lv.2 / Pisces Heal lv.4... Really boring power combination i must say, but i do not have the ovaries to tackle these chapters without enough first aid