MLBH is great, but you need a back up for GMB users.
I say go with Pursuers and the Lightning Avelyn, just don't shoot if they roll through it towards
you, you may hit them, you may even hit them with all 3 bolts and stagger them, but at that range, they will still recover before your reload animation is complete.
Dark Bead's range is underrated. If you have 12, you can chip off about 25% of their health from a distance. Just save 1 Dark Bead for a well timed parry...it'll end the fight.
If you're not gonna buff your weapon, then use Strong Magic Shield.
Dark Bead-->Dark Orb is a decent combo. Just don't give that combo away from the start. Make them think DB is all you have.
Keep in mind DB will still break poise on a GMB user. Use that if they press overconfidently, then backstab with a...Chaos Dagger or something.
Have fun and don't get too confident with Dark Bead. It's an excellent spell, but can be rolled through like anything else.