I did it on the very first boss, the First Warden.
Since you can't get hit I took off my armor as I think it improves the speed a little bit. Maybe not.
This boss can only be hit from the back or when he is off guard. Normal attacks will be blocked by his shield. Always dodge his attacks, even with blocking they will still do some damage
This boss has some easy attacks and the good thing is he does them even if you are far away. First he has the shield bash which doesn't do any damage, but best to avoid it as well. For this one he holds the sword like a spear towards you and then runs in with a bash.
Then there is either a single or a series of 4 swings. Dodge AWAY from him and wait it out.
And the last attack is the one you want to exploit. It's an overhead hit where he hits the ground. Dodge to the SIDE as this attack sends a trail of flames on the ground. While his sword is stuck to the ground quickly run up to him and do 3-4 hits with daggers max. Sword or axe will probably equal 2 hits.
On the last 25% of health he will have another attack: he will just swirl with his sword, I think it's 8 spins. Again: jump away and wait it out from far away.
The achievement should pop right after a cutscene.