A Japanese solo has been recorded after reportedly 10 hours. It is very luck based.
Item: Silver Powder
EVs: 252 Atk / 210 Def / 48 Speed
Moveset: Leech Life (max PP) / Bulk Up / Amnesia / Mud-Slap (max PP)
The AI Bellibolt's paralysis seems to be valuable in putting luck in your favor here and not getting hit, especially in the double action phase. I believe Mudsdale and Weavile may also be contributing defense drops through Rock Smash and Leer later on that may help efficiency.
The strategy from the pinned comment seems to be:
1: Up to 75% of time (debuffs reset)
- Mud Slap x 3-4
- Evenly stack Bulk Up and Amnesia
- Tera Leech Life when health is low
2: Up to ~75% HP (buff reset)
- Mud Slap while Bulk Up and Amnesia x6
- Leech Life when health is low
3: Up to shield down, debuff reset, Chesto Rest
- Bulk Up x6, Amnesia x1-2, Attack Cheer, hit with Leech Lifes
4: Up to the end
- Mud Slap x4 then Leech Lifes
- Just before 35% HP (buff reset), do an attack cheer and Leech Life. In the video this barely left Mewtwo at a sliver and the AI actually finished it.