As the name describes, SDE#0X is a Shadow Experiment made by Ultima (the good one). But it went haywire.
It has 10 forms, the first 3 copies the main characters, then Boomer, then Butch, then Rude, then Ahriman, then Weegee. Then finally himself twice.
There isn't much of a stategy, cause he does the exact same stuff the characters he potrays do. But he does have his own, HP, MP and Moves so here is that.
Form 1: Dark Gabe
HP: 999,999 Oversoul HP: 3,896,909 MP: 999,999 ATK: 999,999
He is pretty easy, just attack him and dodge his attacks, don't heal with the "Cure" magic, cause he will cure, use potions instead. You want to attack the back, or else he will block every move. (Except for overdrives). Watch out for Zantetsuken, which will do 999,999,999 Damge, so be prepared.
Form 2: Dark Brick
HP: 999,999 cannot be oversouled. MP: 699,996 ATK: 999,999
Once again, pretty easy, he will cast Flare here and there, but it'll only do 4,692 HP. He will attack with his yo'yo. You want to use magic, because he will use Flameplosion if you get close, which does 99,999 HP. When he gets weak he'll summon Praestringo, which now you have to deal with that. It only has 9,999 HP, and only does 3,999 HP Max. Once you defeat Brick he'll do Mega-Flare, which does 999,999 HP.
Form 3: Dark Reno
HP: 999,999 MP: 799,999 ATK: 345,678
This one's a little tricky, there is no pattern with Dark Reno, he can throw his metal pole thing very far away, dealing 10,000 damage. His Teleport move is what makes you confused he will teleport randomly, losing your Lock On. He will do his force slash every 15 hits you give him, doing 200,000 damage. Once killing him he does Meteor, dealing 345,678 damage.
Form 4: Dark Boomer
HP: 1,562,591 MP: 999,999 ATK: 999,999
Form 5: Dark Butch
HP: 1,534,587 MP: 999,999 ATK: 999,989
Form 6: Dark Rude
HP: 1,793,999 MP: 999,999 ATK: 999,999
Form 7: Dark Ahriman
HP: 3,999,857 MP: 999,999 ATK: 999,999
Form 8: Dark Weegee
HP: 9,999,999 MP: 999,999,999 ATK: 999,999
Form 9: True Form A
HP: 999,999,999 MP: 999,999,999 ATK: 987,321
Final Form: Shadow/Darkness Experiment Number Zero Extreme
Here we go, not being as strong as the Nexusverse Destroyer Secret Boss located everytime you dash into the barrier 100 times. He still is very strong. Insta-Killing you with a single hit, this "demon" is stronger the Omega Weapon himself. Also... he has 999 Trillion HP. He has many hard to escape moves like Explo-Plosion which is an explosion which explodes within itself and Nexus Ultimus a move which tests your timing of casting Reflect. His other moves are Flame Burst, Mountain Throw, Flame Breath, Axe of Magnus and Huge Ass Boom. When he dies he'll do Full-Cure. (sigh). Once you beat him again he explodes into 1,000,000 bits. And The 100 Trials is created