I have got 42/43 achievements.

Although I still don't know how to unlock the last achievement Metal to the Max, but I find some easy ways to get Bounty Hunter and Shopaholic.

First, make sure you have enough power to destroy the final boss. If your game difficulty is Normal, open Options menu, select Gameplay Settings, then change the difficulty to Hard and destroy Catastropus.

After you load the latest autosave, the next difficulty level will be unlocked. Repeat the steps above to unlock Hell, Heaven and Ragnarok.

The easiest way to eliminate Catastropus is to attack it from behind at Tokyo Tower.

Now you can find additional Wanted bosses on the map. Before you begin fighting, don't forget to change the difficulty to Normal. Otherwise, they can destroy you without effort.

However, if you change the difficulty too early before you can see bosses, they will disappear.

If you face the same situation:

1. Play game in survival mode.
2. Finish the main quest of library.
3. Unlock all items in the shop.
4. Buy every item from shop.
5. You still can't unlock the Shopaholic achievement!

Then I can give you a very effective solution:

1. Save the current game and make a backup copy.
2. Change your game difficulty to Ragnarok.
3. Remove all equipment from your characters and vehicles.
4. Sell all items, materials and equipment.
5. Leave Iron Base, save the game and return to title screen.
6. Load your save slot and enter the base.
7. Make sure you only buy items from Terminal. Don't buy anything from vending machines.
8. Firstly, buy tank supplies from cannon to trait chip.
9. You should wait patiently until the text of "Now Loading" end. Don't press cancel button in this process.
10. Secondly, buy handheld equipment from consumable to defense.
11. You only need to buy one item from each type.

Although I'm not sure which step is necessary, but I got the Shopaholic achievement after these steps.