Start the Storymode from:
Game home > Quest > Story Dungeon.
Begin from the Prologue
Just keep killing those Goblins. You come by them very often so repeat the level or keep playing Story Mode. Pop goes the Weasel as you reach 100 Kills when you play.
Note: You can See the progression of You're kills by Snapping Achievements App.
Dubble TAb the lit Dashboard button on the controller, then Right Trigger All the way down, Snap App > Achievements. Look for Goblins Shmoblins .
Hope this helps!