There are two main keys to getting this, in my opinion: first, make sure that you're up against an enemy you can counter, second, start your attack chain with the counter, since that is the one element that relies on timing/your enemy. Also, make sure you have your bomb active as your item use key - several times I forgot to double check and missed opportunities because Geralt was shooting his crossbow rather than throwing a bomb when I hit my middle mouse button.