For this, You need to reach 70.000 points in Arena and it's only possible in Endless mode. To unlock this mode You need to finish Arena 16-20 maps first. If you reach 70.000 points you can "End Arena mission" in the menu WITH THE SCORE YOU HAVE!
As at the comments at the other guide mentioned, the biggest problem is the game keep crashing. After several attempts and freeze, I made a hard reboot as "UnmatchedBrace" (hold power off button for ~10 secs) and as "Oathiel" recommended there, I turned off the "Blur" in the settings and the font size was on default for me. I did not reinstall the game.
After those - at first try - I could reach 70.000+ points at wave 74.
Luck or else?! I don't know, just want to tell how I got it finally... So for the waves and setup:
You can get 2 EXTRA Major Core slots, if collected all parts of the Abyssal armor (5-5) for both characters. I got only one as you can see, there is the "Moloch" core. If you don't have any, you can put this to somewhere, but keep the "Match Bonus".
Most of the Major core should be at level 3. For Belial, Moloch (and Mammon >> see below) just simply play again their level (only a boss fight) and you will get 1 core for each win. For Hollowed Fiend core you can play the first level again, but that's a whole level... I couldn't get Moloch & Belial core elsewhere.
I killed Wicked Killington 3 times, so I got Wicked Willington core for lvl 3 and with this every killed enemy explodes with 6m radius with heavy damage. It helps a lot.
You can check my guide for that here:
If you want to farm souls you, can do it on level 6 Mammon boss fight, it's around 1-2 mins and you can get 8-9k souls (2-3k souls + Mammon core >> 5-6k souls for it if you are on lvl 3 of that, but sometimes does not appear).
For farming use these cores for extra souls:
Minor core - Void Magus +5-15% souls
Minor core - Ghoul +5-15% souls
Major core - Mammon +2-8% souls
Here is a video (it's faster because I'm almost 1500):
So If you are ready for the Arena - I recommend to try this at least 1300+ - here are my tips:
Always run around and dash (evade) if necessary.
Use the clones (LB + X). They distract the enemy, so they attack them, not you, so you can attack them safely from behind.
I bought the "Vessel of undying" 50k souls + 15 coins. It's an "auto-healer", just in case, if you have souls.
For the secondary weapon the Static is quite good, mostly for hordes.
Keep 1-2 wrath bar (clones for distract) and the Anarchy Form for the 5th rounds (5-10-15th...) those are Boss fights.
While in Anarchy form use X or Y, not the gun. Because with X-Y you can hit more enemies at once.
If you die with one character, with the other keep running around and try to stay alive for 20 seconds to revive the other character.
Check these videos how it should work:
Today I tried to go as far as I can - or as far as let me go without crash -, so I saved (a video for the records) after every 5th round. Unfortunately at 107 "dashboarded" me :( . Here is the last video: