DS2 is really two games. The base parts, everything gets raped by dark magic and lightning. The expansions, most shit is very resistant to magic but is somewhat vulnerable to one of the physical damage types. Also remember that DS2 has mobs go extinct if you kill them enough times (unless you level up the zone) and a couple areas you can level up that are not much harder than the first cycle but then the boss drops something new thats usually powerful. The Sinner is a good example of that.
If you are going full Unga Bunga, then just use one time buffs where you feel you need the lightning. If you are mixing in faith to any degree, then craft it lightning for the core game parts and then switch back to either base or RAW depending on whatever gives more attack at that point.
Bear in mind that the best PvE weapons in DS2 are generally Halberds because they can do multiple types of physical damage depending on what move you use. The BK Halberd is arguably one of the strongest all around weapons of the entire game and includes fire damage. But.... if greatswords are your thing, the one you get from the Last Giant early in the game eventually ends up being one of the strongest in the entire game. The MLGS in this game is arguably the weakest its been in any From game, but still usable. Sinner GS is fantastic, though.
DS2 is the hardest game of the series, overall with all content considered in it, for a few reasons. First is the options for cheesing encounters are much more limited in this incarnation, despite having some of the strongest spells. Second, the optional boss fights are the hardest of the entire series, with the two cats and the tripple red phantom fights topping the list. The runbacks are also among the nastiest in the entire series, unless you are willing to extinct an entire zone to eliminate that (which is sometimes a good souls farm anyhow). Lastly, it has a large number of giant fuck you melee guy type fights for both bosses and hall trash. I would also add that this is a game where sometimes having a shield is mandatory if you intend to melee certain enemies, which is quite unlike most of the other games in the series.
Difficulty is easier in other games for a number of reasons. DS1 was mostly hard because it was a new experience for most people (see also Demons Souls). DS3 had a couple individual ball buster fights (Midir and Nameless King), but was otherwise fairly standard with good farming options with a very broad variety of strategy options. ER took that and added the flexibility (and exploitability) of an open world, with the optional boss Melenia being the only absurdly tough fight in the game.
