I'll share my tips on how to fight Dark Magic, suitable for everyone, so no point investments required, so GMB, VoS etc. I got none of that on my Dex build, so I use the following methods to deal with Dark Magic.
Dark BeadCrest ShieldAnyone can use it and it has good 85% damage reduction from magic. Eith 40 End and RoFaP a full hit of Dark Bead, boosted by Tin Crystalization Catalyst and Crown of Dusk does little damage, somewhere between 150-200 HP max, which can be further reduced by armor and takes around 60-70% of your stamina. Stamina drain can be a problem if you're facing a spammer, but can be dealt with by using Green Blossom before the fight and combining blocking and evading.
EvadingSimply roll through, or to the side of the attack. Unless lag comes into the play you'll be ok with 0 damage taken, even with medium roll.
Skilled casters will fire Dark Bead unlocked which kills this tactic though, so be carefull and don't be ashamed to use your shield. It's better than dying, right?
PurusersCrest ShieldContrary to the popular belief Purusers CAN be blocked, but they'll devastate you, break your guard and leave you vulenrable. However you can safely block 1-4 of them, depending on your Stamina bar size with this shield.
EvadingYou can roll through Pursuers, but they can follow you and hit you from behind. What makes this occur is still unlcear to me, but so far, I think it depends on the length of Purusers following you, so the sooner you roll, the bigger chance they home on you again. This worked for me in 100% in combination with one of the following tactics.
Rolling to sides is viable option, but leaves you open to subsequent spell and/or melee attacks from the caster.
Positioning yourself so that you get some object, like a tree for example, between you and the Pursuers, so they hit that object instead of you.
Retreating from the caster is by far the safest method I've found, because the Pursuers can't catch up with you. You have a lot of time to create some solid distance between you and the caster when he starts to cast Pursuers as they have a very long casting animation.
The caster can run after you to try to attack you with a melee weapon, or to cast another spell, like CSS for example.
In this case, spells are of no issue as you back off from their range before the caster manages to pull them off, or simply block them. With enough distance between you, the caster can lose lock on, thus making his spells miss you and you also get out of their effective range.
If the caster manages to pull a spell on you, or catches up to you, you can easily evade this. Spells by rolling to the side and melee by rolling backwards and still keep your distance between you and incoming Pursuers. Or, you can, once again block, this works especially well if the caster decides to attack you with a weapon.
However at this time, the Pursuers should be near their end.
Roll BS is a very good method, but not everyone can do this and it's not 100% reliable, mostly thanks to lag, however it can be turned into a anti Pursuers cheese by backing away, this usually forces caster to follow you, when he's getting close, roll through Pursuers for an easy backstab. This worked for me 100% of the time, becasue the caster is in sprint and can't turn instantly to repel your roll bs.
Also at this time, when I've rolled through Pursuers after they've chased me for some time, they never turned around to hit me and just continued their flight in the original direction, or dispersed into various directions, but none turned around.
This makes me think that Pursuers lose the strength of their homing ability with time.
Havel's SheildAll of the forementioned tactics work for fast and medium roll builds. Heavy roll builds should probably rely on Havel's Sheild, since they're usually having high Str.
With it's Stability and defences and with good stamina managment on your side + some Green Blossom, just in case, you should be able to block it all.
These works only when lag plays none, or minor factor in the equation. We all know when lag comes, all sort of
happens.
PS:
I didn't try to block Dark Magic with Dark Hand, however I managed to block some type of Soulmass (I can't say which thanks to DLC bug) with Dark Hand for reduced damage and 0! Stamina damage. However this could've been thanks to lag and I didn't attempt to recreate this. Not that it matters, since evading Soulmass spells is easy.