Found towards the right of the second area. You need 2 keys to get to the boss
Save up axes for this boss to make it super easy
Achievement Walkthrough:
TimeStampsMountaineer Gloves: 1:20
Club: 2:25
First Chest: 3:25
Health Upgrade: 4:25
Lever: 6:20
First Key: 6:20
100 Fruit Fence: 6:50
Stamina Upgrade: 7:20
Boss 1: 8:05
Tonic of Fortitude: 8:35
Bamboo: 9:20
Boss 2: 13:00