This achievement has more to do with RNG than anything. Therefore a solution is a bit tricky.
You do not have to defeat the Warlock to run into the Living Creature (as confirmed by a developer from ACE team)
The Living Creature has a random chance to spawn in the "town" areas (the upper most stages of the game) when YOU spawn into it, when the event happens a cut scene will trigger and the Living Creature will come out of the pit and attack you.
At this point in the game you will likely have no items, potions, or upgraded weapon. It is recommended that you use a character with some levels for the health buff. I used a level 20 Monk and the creature still took 85% of my health down.
You can increase the odds of the Living Creature spawning by...
- Selecting the Pocuro park spawn point when you first start up your new session from the menu.
- In the Pocuro Park, if the event doesn't trigger jump down into the pit and completely DIE on the first stage. This will put you into the "Limbo area". Spawn back into Pocuro Park. rinse, repeat.