Craft The World

You can farm PvP wins quite efficiently if you play against a friend and he just keeps suiciding his dwarves as soon as the game starts.
One "game" takes around 3 minutes so if you take turns you will both have the achievement in ~2 hours.

Prerequisites:

100 mana (>150 if you want to be on the safe side)
Magic explosion
Fireball
Play on level 2 or 3 (ice or desert world)

How to kill it:
Mine some blocks just above the lava. When the worm spawns, cast magic explosions on him until his life bar goes to zero (I think it took around 6).
Finish him off with a fireball since the magic explosions will not kill him.

For me this popped on the 4 campaign level after ~20 hours.
It seems that Nightmare difficulty is NOT necessary.