Main - The main story campaign will be the easiest to do, seeing as all you have to do is to help Rain, the town mayor, with her problems, and then subsequently Lord Adam, the local lord who presides over the region of Cotton Hill. There are no demons/fiends within this character's storyline, and so you can be a paladin or not and fight with basic weapons to get through it.
Nightmare - This is a subset of the MAIN questline, wherein you will need to be a full fiend before the start of the second phase of the main questline and be called out by Lord Adam for it. Upon doing so, you will complete the second phase but be banished from the village by Rain, and thus sent back to camp, wherein you will be visited by a nightmare who will take you to see his lord and master, an archfiend. If you take on this questline the endings will change, and you will have to fight through one dungeon, a coliseum tournament, and then against a very hard final boss. You can still gain access to crafting, but not the areas to forage if you have not dealt with Krasz' questline, so difficult will vary depending on where you are in the rest of the story.
Krasz - This questline can be difficult, only because there are several mandatory boss fights within it, of which will be quite difficult, depending on your level and your current equipment, of which will need to be enchanted/specialized due to having demons/fiends to fight throughout. Also, you will have three boss fights with Krasz himself in different phases, of which the last will determine which ending you receive.
Russ - This questline can be considered the easiest within the game, as all you will need is a decent level and equipment to get through it. There are no true demons here, so you will not need any specialty equipment to get to the ending.
Sen - This questline will demand that you have taken a seed of chaos in order to start it, the former can be obtained by completing the first phase of Russ' questline. IF YOU HAVE NOT TAKEN EITHER THEN YOU WILL GET A BAD ENDING AT THE END OF THIS QUESTLINE!!! Afterwards, you will need a high level and powerful enchanted equipment, as Sen will send you down a rabbit hole that specifically deals with fiends and the like.
Nibbles - This questline is split into two parts: the first will see you having to aid Nibbles in building up the town inn, by way of paying him, and taking on certain...jobs for him. Afterwards, the second part of the questline will open, which will involved you going through a number of one-on-one battles within arena that Nibbles will set up for you. The finale one has a demon fight within it, so you will need to craft enchanted/specialty equipment to get through it.
Victory - This questline only opens after the first part of Nibbles' questline is completed. You will have to fight any /fiends during it, and so it can be considered easy enough once you can get to it.
Hammer - This is the most difficult questline in the game!!! The main reason for this is because Hammer will be hounded by the undead, which you will need enchanted equipment to repel, and then you will have to go through a very battle heavy second which will put you against wave after wave of undead and then a boss. Afterwards, you will still have several other bosses to contend with before you reach the end.
Felix - This questline can be considered difficult or easy, depending on your level and equipment, as there are no demons/fiends to fight, but you will have to fight a gauntlet in the third phase to get to the ending.
Whyte - This is the second most difficult questline in the game!! The main reason being that you will have three gauntlets of enemies to fight, one for each ending phase of the questline, and they all have demons/fiends at the end, so you will need specialty equipment to defeat them.
Cinnabun - (This character can only be found in the second area of the forest at night and only if you have 0 purity.) This questline can be considered the third most difficult, if only because there will be two demonic boss fights, which necessitates that you have enchanted equipment to defend yourself with. You also will need to be at 50 Purity to even start it, and have access to the second area of the forest by way of seeing through Krasz's questline.