Why???

15 minutes per run.
23 Radiant fire crystals per run.
3-5 hours of farming per 5* item.

Does that sound good? Well it should. Imagine Firestorm Citadel, but instead of going for hours, dealing with team members falling away.. you just get radiant fire crystals and you can quit easily.

How???

Similar to Vanaduke, we're going to have two roles- the damager and the cleanup crew. The damager is going to deal crowd damage the likes of which you've never thought possible, and the cleanup crew is going to clean up the stragglers for them by bringing weapons and positioning that they can't due to their specialized loadout.

The Damager

Iron Slug
Electron Vortex
Valiance

Step 1: Vortex
Step 2: Slug
Step 3: Repeat 1 and 2
Step 4: Clean up with Valiance

But isn't it the cleanup's job to clean up??? Yes, but we want both parties doing something at all times, and not sitting around waiting. Waiting is dumb. The point is to go fast. The damager is the harder position to fill, as they need specific weapons and strategies.

The Cleanup

Dark Retribution/Dark Briar Barrage
Ash of Agni
Super Nova

Step 1: Spam bombs
Step 2: Clean up with super nova

Like using swords? This position is for you! You can easily swap out ash of agni for whatever sword you want. Like spamming brandish charges! If it helps you clean up, it's game. The only requirement is that you have some form of area damage that you can spam, and that you don't interfere with the damager's job.

But how do I make it happen???

You're probably going for a position as a cleaner so that you can use your brandish. Your first step should be to locate someone with the opposite loadout, which can be done in Haven. Inform them of this guide, and get them to do that. Once you have someone, don't just go once. Go like 5 times. You don't leave after floor one of FSC, do you? No, you do like 5 FSCs because it's farming time. This is the same situation, but harder.

FAQ, that you should skim/read

I'm bad at the game and not ready for this.

Yea so was I when I started, but then I went like 20-30 times, used a few sparks, and wow look at that I don't suck any more. This process is known as "getting good". If you're capable of completing other missions around this one you're ready to start this one and just need to practice it alone a few times. Once you can solo you're ready to speed farm and believe me, it's worth it. If you're not capable of other missions go back to the one after the last one you can complete solo and practice that one.

No FSC is better because I need crowns

Then do FSC this is for radiant fire crystal farming.. but when you need radiant fire crystals then this is the guide for you.

What armor should I use???

Black Kat/Chaos, because there is very little status and that status is poison.

What shield should I use???

Ironmight plate/slime is what I prefer, but you can bring the yellow tortodrone shield for optimal protection... as long as you don't mind a bit of movement speed decrease :(.

How do I fight toxigels???

Just like everything else. Run in, drop a bomb, be fearless. Oh, and sprite skills. Optimally, both parties bring seraphynx, though one seraphynx is enough. How do you rush in to a bunch of toxigels? Well, you use seraphynx's last sprite skill, the one that protects you from damage. Since the attacks are random, you usually only get hit by a very small portion of the quills that the aura can tank.

Is this only the last part of the arena..?? Do I fight everything?

No, run past as much stuff as possible I'm assuming you already figured that out because you're a smart cookie.

Am I forced to fight the final battle without the NPC?

Yes. This is actually a blessing in disguise, as the NPC moves enemies around quite a bit- not something you want when vortex+slugging through things.