Good "speedrun" course:
Open junkyard, beat goat and crusher, go to lower L, beat Matron & Bucktooth, and The Billy Goats Gruff.
Go to Isle, beat Megadrill.
Open abandoned labs, defeat Big Foot.
What I consider "beating" the game:
Completing all story quests, side quests, find all plans, defeat all bosses, and craft all four cape suits.