Eldest Souls

Whatchdog is the first and easiest boss in the game, this achievement must be done in NG+, in the first run the tutorial force you to get hit. The boss has only 3-4 type of easy to avoid attacks that will repeat for the entire fight.

Below you can see the fight, it take too long because i used the NG+ of the no abilities run and i forgot to power up the character, with the abilities it will finish really fast!