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This board is dead and I'm not sure how many people end up going so far as to Platinum this game (.04% according to PSN), but here are a couple of tips for completing this. It's insanely difficult to beat the Demonic Core Stage 6 on Nightmare difficulty in the Arena without the right character/strategy (and honestly a bit of luck). This took me at least a couple of solid hours of frustration.
First of all, use Hugo (I believe Adol is also do-able, but I haven't tested him much at this point). It's possible with other characters, but Yunica and Toal are going to get chewed up bad when you get to the last mob that has the Glaregraves (the purple knights). Besides, there is an easy strategy you can use with Hugo that also tends to work well on the other stages if you are having trouble. It works especially well on this one, and I'll explain why.
With Hugo, immediately use your Wind Shield charged up and go to the upper-right corner of the room. In the PS4 version, all enemies spawn in the lower-left hand corner (I think other versions spawn more sporadically, making this much more difficult). You can safely use his shield the entire fight until you get to the very last screen. His long distance shooting at the right spot in this corner will keep him out of most of the enemy's notice while you shoot the hell out of them. Too far and you won't hit them, so it's toward the corner but not all the way. A lot of the enemies will notice you and come for you as you hit them, but you should be able to finish off groups of them before they start to lay into you. The enemies that do not take notice are easy sitting targets, and you can just keep shooting them until they are clear.
Don't go for items and SP unless absolutely necessary (they are mostly going to be at the spawn point). With your shield up and charging as you shoot away, you'll deflect the majority of the damage.
The mobs that contain the claw-things that spin at you and confuse you, as well as the tentacle beasts, will be more difficult. You'll need to move around and jump constantly, and shoot with your shield up. If you get confused, try to hold off on finishing the last enemy until it wears off. You'll also need to become accustomed to moving while confused to be safe - otherwise, you'll get cornered and defeated easily.
On any screen with stationary enemies (the pink things that send out razors, the ones that pull you in, and the ones that send out spinning projectiles) will always stay in the corner, allowing you to chew them up at a distance.
Your Burst probably won't charge until the later mobs, and that's ok. Save this for the last couple of screens where you run into the mages that shoot homing missiles. When you get there, try to use your Burst as they spawn and move around to clear out as many as possible.
On the last screen, you will almost definitely need to use Burst to clear the mages out with the Glaregraves running around, and you may need to take some damage or bide time until it charges again. This is the most dangerous screen by far. Mages are your top priority - keep your shield up and take them down. Once they are gone, switch to your Trap Mine skill and use charged mines while running around to avoid the Glaregrave attacks. When they turn dark purple, be especially careful, as they use a follow up attack that will kill you in one or two hits - keep moving, and each time the Trap Mine charges, lay it and press the skill button again immediately so it goes off. Keep this up, and you'll win.
The reason the other characters won't do as well is because Hugo continues to move as you lay the Trap Mine, and Thunder skill is the only thing that hurts the Glaregrave knights to make a difference. Yunica and Toal both stop while using their Thunder skill, making them easy targets. It's possible, but incredibly difficult, to win with them because of this.
Anyway, hope this helps some of you! Boss Rush is probably harder, but godspeed to you, and git gud.