I'm new to vanaduke, only 25 tokens in fact, but I fancy writing a guide.

My setup as a gunslinger:
Vog set (I have a nameless set, but fire resist is so much of a priority...)
Grey owlite shield
Leviathan blade
Polaris
Hail Driver
Sentenza (might consider the awful argent peacemaker because sadly sentenza does half damage in the 5th stage for no reason at all...)

For FSC, the levels before the boss, all you need is elemental weapons. Here are some recommended weapons to bring:

Divine Avenger - Strong, deals elemental and people use it for magic tricks in the dungeon =p
Polaris - Strong gun, deals elemental and good knockback
Leviathan/Cold iron Vanquisher - Useful throughout fsc and for the boss
Argent peacemaker - for the boss

Theres many other weapons but I would say this is the top 4 to carry with you. Other good weapons include:blitz needle and shivermist.

Ultimately, you only need two weapons for the boss. A normal sword and a normal/piercing gun.

Any shield would be good. The best being those with good health and fire resist.

Armour, I recommend anything with good normal defense, maybe shadow defense and fire resist.

You arrive at vanaduke...

In a team of 4, I like to give everyone roles. 2 people will be on the attacking team and 2 will be on the support team. The attacking team takes care of vanaduke, simple. Support takes care of the watering the fireballs, mask and lava. They also take care of the slag guards.

First stage: He spawns 4 slags and the fight begins. You either just hit and dodge him or keep him frozen all the time. There are two tactics that I like. One is the hit him from one side then when he strikes move to the other side and hit him and carry on. Two is to have the person whoever is aggro on him to keep running around him, so everyone can hit him.

Second stage: He walks back and a mask appears. Have one person ready to throw water at it, while the other 3 starts hitting it.

Third stage: Most beginners die here. It's the same as stage one, except he has 4 fireballs around him. This is where the guns begin. His fireballs will revew itself, but it's not a bad idea to water them because it blocks bullets. Support will take care of the lava and slags. While the attacking team keeps vanaduke busy.

Fourth stage: Same as 2nd stage except a lot more fireballs thrown at you.

Fifth stage: This is where the real fight begins. It's similar to stage 3, except he has more fireballs and spawns 2 slag guards. What's important at this stage is teamwork. Support must now take care of the lava AND the slag guards. They will charge at you and hurt you a lot, so having 1 or 2 people to keep them away from the attacking team will help a BIG DEAL. If you kill them or if vanaduke respawns new ones, hearts will be dropped. So some if you team mate dies, don't revive him until the hearts appear. The attacking team will make sure vanaduke is away from your team mates. The last thing you want is for vanaduke to start striking the support team, WHO is support YOU! So the best solution is to lure him away and start shooting and dodging him. Keep this up and soon he will be dead.

Reminder and tips:
Save 3 heals, 3 poison, 3 remedies and 3 vials of your choice. Poison will help you deal more damage, my preference.
Try to save the health pills for at least the 3rd stage, but do heal.
Try not to revive your teammates unless absolutely necessary.
Revive those who are strong and last long.
Kill or wait for the slag guards to die in the 5th stage for hearts.
Kill the slags in the beginning stages for hearts and less disturbance.
Always water the lava! All of it!
Have someone to be ready to constantly water the mask.
Distract the slag guards in the 5th stage, it's vital for victory.

I may have missed out some stuff, but this is what I can recall. I'm new but I'm starting to get the hang of it. Hope this helps :)