respond well to magic in the sense that most times people are using powerful magic on them, they have a good catalyst and high INT.
If you have melee, your best bet is to tank the hell out of them... I did this in my first game, as my character only had about 24 (at best) INT at that time and nothing was really doing a lot of damage. The 2 handed lightning spear while wearing stone armor (stolen from the speed run) was more than enough to wreck them.
As far as your magic weapon question, yes it adds magic damage, but the main benefits of magic and enchanted are that they scale with INT -- so if you don't have a high INT, the weapons are probably going to be weak when you use them.
With that said, one of the best things you can do with ANY weapon upgrade is to do the upgrades on a weapon for which your character is suited to using and you like the moveset for. This is why I have a lightning iaito+4 in addition to my lightning spear+5.