To run a server for Eaglercraft 1.12.2, you should run a standard Paper 1.12.2 server and use the Eaglercraft EaglerProxy or Waterfall plugin to allow web-based connections.
jadenacoder/Eaglercraft-1.12.2: This repository is the ... - GitHub eaglercraft 112 2 github upd
The update brings much more than just a higher version number. It shifts the entire technical framework of browser-based emulation: To run a server for Eaglercraft 1
Running a game as complex as Minecraft 1.12.2 inside a browser engine demands significant CPU and RAM resources. If you experience low frame rates (FPS) or stuttering, apply these tweaks inside the client settings: eaglercraft 112 2 github upd