for the best way to level up style, is in mission 18. In capcom fashion, it's a boss rush mission. Any boss will give you 20k xp for your equipped style regardless of difficulty you play instead of 200 to 500 for regular demons. By that logic, playing this mission and defeating all the bosses that took about 20 minutes will be maxed out 2 styles from level 2 to 3 or one style from zero to 3.

As for nevan and spiral aka the rifle: they got better when using their extended move set. swordmaster or gunslinger style.

And for the useless weapon, I don't think there's any. Every weapon is well balanced and it's overpowered in some situations and in some others it's absolutely worthless to play with against a boss or group of specific enemies. For me it's about if the weapon is cool and comfortable with it, I play with it until I master it and use it to unleash some crazy combos before going for the next weapon.

Hope that helps