As advice for Vagrant Story, weapon affinities don't matter as much as weapon types and enhancements/buffs so there really is no reason to grind your weapons against certain enemy types. The fire phantom for example can easily be killed if you enhanced/buffed your weapon with ice/water. If you're great at the QTE parts of the game, you could also keep chaining any enemy because each hit increases the damage by 1, thus the damage sequence would look like this: Miss, 0, 1, 2, 3, 4, 5, 6, 7, 8... IIRC, by the time you reach the fire phantom, you should already be able to use Break Arts, which is another alternative if you can't chain nor enhance/buff, and Break Arts are pretty powerful and deal good damage in exchange for HP.
As for the variety of weapon types, there are the 3 types that deal the damage: Blunt, Blade and Pierce. You should carry one of each and use them for whichever is most effective against an enemy's weakness. The reason there are a lot of them is for players to find which works best for their playstyles. Not all Blunt weapons have the same reach, Break Arts and QTE timing. I don't think it really should be an issue as to why we are given a plethora of weapon choices seeing as the Souls games does have lots of choices as well. I also found that being able to target different appendages great: Is the enemy a spellcaster? You can use a spell to silence the enemy OR keep attacking the enemy's head to achieve a similar effect. If there are multiple enemies and you'd like to deal with them one by one, you could attack the legs of the others to half their movement speed.
Plotwise, it's typical Matsuno political-intrigue and power struggle. Not surprising as he was a student in foreign policy. IMO, games could use more of these kinds of stories. By the way, here's a video of some guy defeating the fire phantom just to show how easy one can beat the fire phantom:
https://youtu.be/E9UqsDXb1Dw?t=382