There are three ways you can get the game. Unfortunately, an app store account is required if you are on iOS.
QooApp allows you to bypass Google Play Store restrictions, but if you want to buy an in-app purchase, you will need a Japanese Google Play account. Attempting to buy things with an overseas account will result in a "You do not own this" error.
Method 1: Google Play Store
2. Create a brand new Google Play account with the VPN on. You must create this account ON YOUR PHONE.
4. Redeem the Play Store Giftcard to your account. You can now leave the balance to rest or spend it on the 120￥ "account-recovery link" purchase.
5. Download the game (obviously?)
 If you do not add a payment method, your account is marked as 'free' and is shown whatever country's store the account's IP comes from. Locking your account forces you to the Japan store and in turn, allows updates without a VPN. You can also reuse this account for other games.
Giftcards with a balance of 1500￥ or less expire after a year.
After Step 5.
Note: Play Store accounts only accept giftcards whose currency belong to the same country as the account. For free (non-region assigned) accounts, those automatically become registered to the region the giftcard originated from. (So if that giftcard is not in Japanese Yen, well, you know what happens; wrong app store country)
Method 2: QooApp
1. Go to QooApp.
2. Find the game you want to download and go to the store page and click "Download". Alternatively, just click here on your phone.
3. Find the game you want to download in the QooApp app, and click "Download".
Method 1: Apple App Store
2. Download iTunes for your Mac or PC.
3. Switch to the respective country's iTunes store. (You can do so by clicking the flag at the bottom right corner of the page)
4. Create a brand new iTunes account with the VPN on. When you create your account, you can put literally any name and address as long as it is valid.
5. Log into your phone using your Japanese iTunes account.
6. Your app store will prompt you must switch countries. Say "Yes".
7. Look for the game you want to download, and follow the usual steps. You may not be prompted to enter address information until this step.
Note: App Store accounts only accept giftcards whose currency belong to the same country as the account. So you can't load an American account with euros, or a Malaysian account with yen.