Early reference error. Early mod classloading of NMS classes is causing sponge mixin to fail.

Logs:

https://gist.github.com/colinrgodsey/aa77126abbc49f46157e798700f3bea2

it doesn't appear to be a patching order issue- it looks like you inadvertently reference EntityPlayer before the FML patching phase is complete. I believe I fixed it, submitting a PR now.

Unless the Sponge team can provide info on how to fix that, the fix would need to be on the Sponge side.

The mod is a standard Forge CoreMod, and loads very late in the cycle (SortingIndex of 1200). If Sponge hasn't gotten in there to make it's patches yet, there isn't much I can do from my side. Looking at the Sponge code they do not appear to supply a SortingIndex for their CoreMod... I think this means they are likely to always patch last, which is probably the source of the issue.