Pretty easy achievement but maybe this helps someone!
To perform a headbash you will need to tap "Y" once you've jumped in the air with "A". The headbash treasure chests have bullseyes on them, headbash every chest in the level for the achievement.
If you're struggling to find them all, throughout this level there are hidden areas accessible with the timed high jump. Tap "A" just as you land a bounce and you will jump super high. Smash the boulders for gems.
- The first hidden area is on your right when you first learn to double jump up the cliff
- The second is on the right immediately after you enter the ice cavern
- The third is after you defeat the moose on the ground level of the ice cavern, you will see the ledge on the right
- The fourth area is behind the Return Home portal right at the end of the mission