The FAQ says that it must be at least 253BC. You've clearly cleared that part of the requirements. It also says that two different generals must lose a large battle against either Hayasdan, Baktria, or Pahlava and apparently one of the generals have to die in the process of the battle. It says nothing about the armies actually needing to have cataphracts in them.

What unit size are you using? If it's huge, large, or even normal, I don't think that 250 units would be considered large.

So grab some stacks consisting of at least 10 units plus your general. Go east/northeast and start looking for large enemy stacks. I imagine this would easier on harder campaign difficulties. I suggest trying to lose without doing an autoresolve.

It's easy to lose a battle like that. First off, deploy your units as far away as possible from the enemy. Secondly, start making them run around, being in very bad formation. The enemy will probably be walking towards you, try to get to exhausted status for all troops. And make sure your general isn't too great either. The less hitpoints for the sacrificial general the better, but better morale traits is ok since you want the general to be fighting till the end and not run out when his bodyguard gets down to only a few units as he usually does in my own campaign battles.

When the enemy comes, keep your armies running around. Turn off all fire at wills. When you see the enemy, throw your general to the middle of their ranks, preferably some spearmen if you see them, and tell him to either stop with guard mode or just let him fight. Try to get him surrounded. The other guys can just sit there doing nothing if you want, but try to get close so the general gets the morale bonus for having friendly units nearby. Once the general dies, the battle should be good as lost. Congratulations! You just lost a battle, on purpose! You masochist.