I am using a popular game using Cocos2d-X for testing.
Download link
Game name: 放开那三国
Package name : com.sincetimes.fknsg

    cd jni
    make install # this command will push compiled .so to your android device

NOTE: you need to modify the NDKPATH variable in file jni/Makefile to the actual path of your NDK. I am using NDK r15c.

    cd .
    npm i

    cd . 
    frida -U -f com.sincetimes.fknsg -l _agent.js --no-pause

This script will list all encypted assets files in the package, and try to decrypt them, and dump decrypted files to your Android device.