You might receive an error that the app invalid device or client, in that case you have to do the following:
1. Install the MOD first.
2. Download and install LuckyPatcher from LuckyPatchers.com.
3. Click on "Dragon Slayer"
4. Click on "Menu of Patches"
5. Click on "Remove License Verification"
6. Click on "Auto modes"
7. Choose first option "Auto Mode(dex)
8. Click "Apply"
9. Wait the patch to finish, launch the game and enjoy...