To get this you must first purchase the upgrade called 'Repayment'. It is on the evil path level 0. The description of how to counter is there but just incase it reads...
"Press 'LT' just before an enemy attack to counter. Press 'X' or 'Y' after countering to counter attack"
This takes a bit of patience to get and can be achieved on a single play through with no 'grinding' as such.
A good area to grind for this is at the end of 'Violence' where there is a large platform on wheels you have to push up the hill, the second you move it 3 Undead Minion's will spawn and continue to respawn untill you move it to the top. Here is the best place to sit tight and keep trying, there is even a health fountain nearby if you get low.
Remember you have to get 20 kills from a counter attack, not just counter 20 times so press 'Y' and not 'X' as it will get this done quicker.
Relics: If you are on the unholy path at level 3 or above and have found the relic 'Hoarders Purse' this will increase the time for a counter attack to be executed.
Another relic that will help is 'Filippo's Rage', when it is at level 2 it will Auto counter an attack meaning all you have to do is hit 'Y' or 'X', but this requires Unholy level 5 or higher to equip.