So you dont have sponge forge installed, instead its a mod that uses Mixins. That mod is responsible on how it uses mixins as well as fixing any issues.
A quick skim though your crash report, I cannot see any references to the mod crashing. But I do have a program called Mixin Detector
That is designed to filter your mods list to just ones that use Mixins. The culprit of the crash will be in that list. Sadly I cannot help any futher unless I find the exact mixin file in your crash log. Mixin detector has the ability to filter down the list even futher, one of which is by mixin file