Tainted Grail: Conquest

I would recommend one of the Children of Morrigan classes for this, which are Berserker, Wyrdhunter and Pathfinder, as they have the Stun card in the base deck. This is obtained against the Faceless Hunter enemy, who will deal 2 attacks more per turn than the previous, unless it is Stunned. They can be found as the "Stray Monster" encounters in the first area sometimes, or mixed in with other enemies. If there are other enemies, especially Abominations, kill them so that you face the Faceless Hunter alone and it won't die by accident. Once you are there, just spam Block/Block type cards or other buffs until the enemy is going to deal 10 blows (will take 5 turns). This will probably cost you a significant chunk of your HP, but that doesn't matter if you don't intend to complete the run, or are playing Berserker. Once you see the indication for 10xY-Z above its head, play any card or item that causes Stun and this achievement is yours.

If you want to try and improve your chances of drawing a card with Stun, you can

1. Keep your Wyrdcandle burning high, so any Tainted Force cards that spawn have a chance to Stun the enemy.
2. Include the Stunning Shout, Random Attack, Crippling Attack or Stunning Blow cards in your deck, as well as the basic Stun.
3. Try and keep your deck small, only adding the above cards so when the time comes, you are more likely to draw a card you need
4. Use the flute that has the 50% chance to Stun from the Pied Piper NPC and pray to RNGesus that it works