Start up a game of Crossroads from the "Play" menu. None of the options matter (except picking Karlsnor obviously) so do whatever you want there. When you can control your character, talk to the Healer and pick whichever enemies you want to fight as this also doesn't matter.
When you're done with the enemies, speak to the Healer once again to get the main quest. To make this quick, spawn the enemy base camp right next to yours. Now run over to the person with the "!" over their head and talk to him/her. Again, pick whichever and however many enemies you want as this doesn't matter.
After defeating the spawned enemies, you will now have to choose a Boss to fight. This is the option that matters! You will be given a list of three to choose from and you're looking for "
Goblin King". If you don't see him there, click Randomize until you do. Kill him after he spawns.
You
should unlock the achievement right after defeating Goblin King but this game seems to have a delay on most achievements (at least for me) so it shouldn't take too long after to get it.
*BONUS*: Now finish up your Crossroads adventure by opening the chest and returning whichever item you picked to the Healer back at your camp. Spark will then appear and create a portal for you after speaking to the Healer. Head into the portal to also unlock:
If you managed to do this without taking ANY damage, you'll also unlock:
Even if you took damage, as long as you didn't die, you'll be 1/5th of the way there for this one as well: