Which magic set should I choose?
I really need some help with choosing the right magic set for my planned build. My current plan is:
Combuster
Acheron
Blitz Needle
Barbarous Thorn Shield
Shivermist Buster (maybe, in case I need a bomb at some point)
The problem is that I don't know if I should go for the Chaos, Divine or Grey Feather magic set. An endgamer recommended the Chaos set, especially for farming Firestorm Citadel. I'm just concerned about the status resist setback there is. I don't want to waste my orbs on a wrong set ^^
I know that Acheron isn't needed for Firestorm Citadel, but it's a nice way of getting there.
Any recommended tweaks are always welcome. I'm a hybrid sword/gunner by the way.

Since you're a hybrid sworder/gunner, you might find my detailed sword guide helpful. Combuster + Acheron + Blitz is a classic selection of many of the game's most powerful weapons.
I would recommend against Shivermist Buster. It's attractive to beginners, because freeze immobilizes monsters. But freeze also hinders tactics such as kiting, knockback, shield-bumping, etc. Expert players rarely use freeze. If you want a bomb, I'd recommend Voltaic Tempest (crowd control), a vortex bomb (crowd control), or Dark Briar Barrage (great against fiends).
My sword guide has an armor section, but it doesn't mention Divine or Grey Mantle. For more discussion of those options, you might read the Other section of my armor guide. Some key points: You don't want both pieces of Divine (because normal protection is important). If you're going to get one piece of Divine, then get the Veil, for its fiend bonus.
How far away from making your armor choice are you? Can you wait until you have a better idea of how dangerous statuses are to you? Also, keep in mind mixing sets such as Divine Veil + Chaos Cloak.
I'm quite far away from my armor choice right now. I'm currently 2nd tier and moving up to 3rd tier (I only need to upgrade my bristling buckler). I just like to plan ahead so I don't waste time on collecting the stuff I want long after I should have.

Planning ahead is great. I always advise players to plan in terms of 5-star items.
But the nice thing about the Magic line is that you don't have to make the final decision until literally one minute before you upgrade. So watch how you perform against status threats, to see whether you should stick to your plan.
The answer really depends on how well you play....
Do you get hit a lot and hate statuses? Owlite and Divine are the safest sets since they resist key damage and status types.
Are you good at dodging and would like a boost in attack power? Chaos is pretty good but you have to be able to predict nearly EVERYTHING to keep from getting hit by a status. If you get hit by a status in Chaos it will last a VERY long time till it fades (or you use a remedy capsule).
For farming the FSC I would suggest the Perfect mask of Seerus and the Vog Cub Coat. This set offers key resistances to Fire and Elemental damage. Most monsters in FSC deal Shadow damage but they should be fairly easy to fight (Zombies), Vanaduke on the other hand deals lots of Normal and Elemental fire damage. This set is balanced with keeping you alive and letting you deal respectable damage at the same time.