You have to kill them using attrition.

Preparation 1: Destroy all enemy troops whilst engaging the kataphrakts with around 3 full units of expendable spearmen.

Method 1: arrow them as much as you can with horse arhcers, then use cavalry to charge into the exhausted katanks, if you feel you don't have enough cavalry to fulfil this task, then retreat and attack next year.

Method 2: Bring 2 units of armoured spearmen, engage the katanks with one group of armored spearmen after your suicide spearmen have finnished occupying the katanks, then charge into the rear of the katanks with your other group of spearmen, I mean get them quite them a distance away, beging charging and sandwich them. If you are churning out spearmen somewhere, box them in and after your formation is broken put all 4 units of spearmen into engage at will mode and let them do their work.

Method 3: Bring out 2 units of high morale heavy cavalry and charge into the rear of the katanks occupied by your spearmen early on in the battle, hope they win a glorious victory and slay the kataphrakts. If either the spear or heavy cavalry break, charge in with your 2nd group of cavalry to lance the scatterred kataphrakts, then retreat and charge again. Begin doing this with both groups of cavalry and get other units to try and prevent the kataphrakts from reforming to help the charging heavy cavalry. This is good against defence lvl 15 jedi kataphraktoi as it may be the only way to kill any of them.