For this achievement, you can easily obtain without playing the main game.
In the menu screen, select “challenges” and select the first level. You’ll spawn in with 5 newbies and 15 tokens. Buy as many as you can, and dump as many as you can into the sacrificial pit. Repeat this process once more by restarting the level and you have your No Mercy Achievement.
It is important to note that this achievement is cumulative, and does NOT need to be achieved in one run. I hope this helps!