Alex Kidd in Miracle World DX: All 15 collectibles (Collector Achievement Guide)

This guide will quickly show you the locations of all hidden collectibles needed for the “Collector” achievement. You need to get all 15 of them in one playthrough. Activating infinite lives will not prevent you from getting the achievement.

1. Level 1 – Mt. Eternal:
   In a block to the right of one of the stone villagers.
   
2. Level 3 – Lake Fathom:
   Kill the 2nd octopus.
   
3. Level 4 – The Island of St. Nurari:
   In a blue block soon after the 2nd lava pit.
   
4. Level 5 – Namui Lake:
   Lose the helicopter and fall into the water at the start of the level. The collectible will be in a block a bit further.
   
5. Level 6 – The Village of Namui:
   Talk to the dog and the collectible will appear.
   
6. Level 7 – Scorching Pathway:
   In a block right above the first circling flame.
   
7. Level 8 – Mt. Kave:
   In a block under the pink block with a star on it.
   
8. Level 10 – River:
   Lose the boat and drop into the water. The collectible will be in the bottom line of blocks right before the end of the level.
   
9. Level 11 – Taiyo Desert:
   Quite early in the level, in a block on the ground between two circling flames.
   
10. Level 12 – Bingoo Lowland:
    Buy the helicopter in the shop at the start of the level and get past the first two lightning clouds without losing it. The collectible will be right after that, in a line of blocks on the top of the screen.
    
11. Level 13 – The Radaxian Castle:
    Talk to the first knight you encounter and the collectible will appear.
    
12. Level 14 – The City of Radaxian:
    In the topmost block above the 2nd circling flame.
    
13. Level 15 – Toxic Numachi:
    In a block above a pair of green blobs.
    
14. Level 19 – Janken’s Fortress:
    In a block next to a circling flame, before a stone thrower.
    
15. Level 20 – Janken’s Castle:
    During the underwater section, the collectible is directly visible in the spikes room.
    

There you are. You should get the achievement as soon as you pick up the last collectible.
Enjoy your full collection!

Thanks to kapinou for his great guide on how to get Collector achievement. all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.