Knights of Pen & Paper 2 Deluxiest Edition

The achievement description is not fully accurate. You only have to make an enemy suffer Sudden Death. Normally Sudden Death occurs when an enemy is inflicted with all 7 status conditions (burn, confusion, poison, rage, stun, weakness, wound), but with the Weapon Rack shelf in the game room you will need one less condition for this achievement.

Go to any town and purchase 15-20 bombs (just to be on the safe side). Now head to the Grinding Farm and battle a single Scarecrow enemy. Keep using bombs and with some luck the Scarecrow will eventually suffer Sudden Death. The reason for battling a Scarecrow is that, provided you are decently far into the story, its stats are high enough not to die from the bombs and low enough to not recover from conditions on its turn.

Of course this is best done with a party of 5, as it will allow you to use more items in between enemy turns.

It's quite easy to get it against an enemy with lot of HP like a dragon.
Just craft 20 giant bombs and use it on each turn: monster will get a random effect each time.
As soon as he will get all of them (confusion, stun, frozen, poison, burn, rage, weakness), enemy will die by sudden death, and achievements will unlock