Add this to your proguard:
-keep class com.google.ads.** # Don't proguard AdMob classes
-dontwarn com.google.ads.** # Temporary workaround for v6.2.1. It gives a warning that you can ignore
EDIT 2020:
For some time now, the Google Mobile Ads SDK has distributed its own proguard rules that get manifest merged into your app, so you don't need to add anything yourselves. The merged rules also keep around methods from third-party mediation adapters that are needed to make mediation work.
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