This was simple android challenge, in short

Decompile to smali via apktool > search for "flag" string > open smali file > base64 string hardcoded > decode base64 string > get flag

We already wrote more detailed on our github.
[Phantom CTF Android category writeup](https://github.com/FrigidSec/CTFWriteups/tree/master/PhantomCTF/Android)