This achievement is a horrible grind. I'm very bad at this game and even I beat the last boss within 30 deads, so 500 is a lot. Gladly it works to just go into the dungeon and then press "start" and go back to the city. Makes this achievement a littly little bit more doable, but still horrible. You can reset around 5/6 times per minute, which means you would still spend over an hour doing this to die 500 times.
I would highly suggest combining this with the achievement for 500 meat and if the boss is close, kill it for the meat.