Adding LiteLoader - ImpactDevelopment/ImpactIssues GitHub Wiki
LiteLoader is a free mod loading system for Minecraft versions 1.5 to 1.12. As Impact currently only has support for 1.12 and above, this guide will teach you how to install Minecraft 1.12.2 on LiteLoader.
Install Impact normally (without Forge) with the instructions in at Installation.
You can find the downloads for LiteLoader here. The version we'll be using is 1.12.2, from LiteLoader's website.
Run the LiteLoader installer and create a new profile, choosing Impact under "Extend from".
If using Impact 4.3 or older, follow these instructions
Find the new Impact+LiteLoader combo JSON that the installer created, and modify the last argument to read as follows:
You will need to modify the LiteLoader jar and the JSON itself to remove the outdated Mixin dependency from LiteLoader.
Instructions
Delete the "org" folder in the root directory of the LiteLoader jar, it should only contain Mixin so this will be fine. Mine is already deleted.

This will prevent the LiteLoader jar from being "fixed" and having the Mixin version restored. Below is what it should look like once removed.

Now you can open the Minecraft launcher and run the combo profile as normal.


