This is unlocked once you have found 14 stamps.
You will be on top of the volcano for the last fight against Boss Wild. It's also very easy compared to the first.
He will be sat atop a huge column firing lasers at you. What you need to do is head towards the markers and hit the switch to open the eyes of the faces on the rotating column. Once you've done this, arm yourself with fireworks and shoot the eyes. Repeat until you destroy the column. There are smaller enemies to contend with, but slam them out of the way or avoid them. You won't have too much problem.
Once you've been awarded the Master Stamp, you will unlock the achievement.