[11:52:47] [main/INFO]: Loading tweak class name net.minecraftforge.fml.common.launcher.FMLTweaker
[11:52:47] [main/ERROR]: Unable to launch
java.lang.ClassNotFoundException: net.minecraftforge.fml.common.launcher.FMLTweaker
    at java.net.URLClassLoader.findClass(Unknown Source) ~[?:1.8.0_45]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_45]
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) ~[?:1.8.0_45]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_45]
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:106) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_45]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_45]
    at java.lang.Class.forName0(Native Method) ~[?:1.8.0_45]
    at java.lang.Class.forName(Unknown Source) ~[?:1.8.0_45]
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:98) [launchwrapper-1.12.jar:?]
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_45]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_45]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_45]
    at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_45]
    at info.mineshafter.GameStarter.main(GameStarter.java:61) [ms-starter.jar:?]
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
 

I need a fix asap

1.12 is no longer supported on this forum.
Please update to a modern version of Minecraft to receive support.