Guide to Fix Dex Issue ( UNITY )

STEP- 1

In Build Settings switch to build  System to Gradle.
	

STEP- 2 

Player Settings => Other Settings  under configurations
Scripting Runtime Version  => .Net.4.x
Api Compatible => .Net.4.x

STEP - 3

Install Resolver and Run Resolver.


STEP- 4

In Android  => plugins => Android.Manifest.xml In <application> tag add following line to solve android Dex-Merge-Exception 

android:name="android.support.multidex.MultiDexApplication"

After the starting tag add following line of code with the Admob App ID
<meta-data android:name="com.google.android.gms.ads.APPLICATION_ID" android:value=" —Id here—”/>