You may want to consider Shivermist Buster. While I don't use my shiv a lot (not a big fan of freeze) I had my best fiend moment with a shiv. My partner died within a "kill zone" and I had left the entrance barrier with only 1 hit and no pills (he had a pill, but I couldn't get to it). I stepped through and dropped a shiv and walked out. Charged my Combuster and stepped in where the fiends couldn't hit me (since they were all facing the barier) and single shot ~10-12 of them. Really cleared out the room. You couldn't have done it any other way really.

I would also go with a shadow shield if your plate shield doesn't hold up. I have Crest of Almire and it works really well. My brother has Dread Skelly. You really can't go wrong with either, but I tend to think shock is the worst fiend type of them all. Although freeze and poison can be just as deadly as shock and fire.

Also, just a thought. If you want to do FSC or fire fiends and don't have armor for it already, this is going to be useless there (extremely high fire weakness). I actually use Crown of the Fallen + Snarbolax for my fiend outfit to get a neutral fiend damage affect with no bad negative resistances. I would not suggest running double fallen gear since that is a huge loss in damage output. If you don't want to get snarbolax, you can always get fallen + Valkyrie for a more neutral set.

i suggest a shadow shield... remember they deal mainly shadow damage and your shield takes a beating
i have a skelly shield and i can take a bit of pounding, Crest of Almire is also an option (and i advice using this in shock stratums actualy) alltough it has lower normal defence (if i'm right it should hold on a bit longer against shock fiends)

the gun, i use the same (i was hoping that OOO would make it pure piercing like the antigua and make a complete new line for elemental or for the piercing) some people say the blitzneedle but that will just make you targetpractise for devil-it's and you will shock to death without ever getting the chance to hit a second one.

the sword, eighter one you pick doesn't matter realy. i would advice to go for the BTB for its charge (can be of some use against fiends unlike FF) i just dislike the piercing weapons (the combo actualy) so i'm stuck using eighter my DA or levi.

or use the snarby bomb :D i hear it wreaks havoc on those litlle buggers haha

and on the armor, eighter shadow would do. Valkyrie is focused pure on fiends. think you'll be better of making a divine hood (fiend medium, ellemental/shadow) and a normal/shadow armor (i use snarby in fiend lvls but that might be out of the question)

last piece of advice, and lot's of people seem to forget this. greavers (the bat like status mist producing hellspawns) will stop their attack (charge) when you hit them (be it with a sword, gun or bomb) use this time to get away from other things around and draw them to you (so they won't surround you)

sorry for the long text haha

May I suggest another type of fsc loadout?

Rss
5* VT
Electron vortex.

You already have an rss, which dps's everything in fsc perfectly fine, no need for a spike shower. Electron vortex clumps together enemies rather well, both for your teammates and your rss. With some boom trinkets, they also deal a decent amount of damage. Solo, it only takes 2-3 vortexes to kill zombies. And you can combo rss for even more damage. With max ctr, you can do something like vortex>rss>rss>vortex.

I go full mad bomber, but if that is too dangerous, I suggest hybrid. It gives you a slightly positive fire resist, if I'm not mistaken.

Volc/mad hybrid
2 E. Boom dmg trinks

Max'd ctr on heated bombs, and max damage.

At the moment I am using something similar (Valkyrie Mail, Crown of the Fallen). The Fiend and Fire bonuses/penalties cancel each other out. Anyway, I still find T3 Fire Undead/Fiend levels to be tricky (particularly when solo). The fire really is lethal if you get hit without strong fire resistance.