I haven’t used MinGW for windows builds so can’t say for sure what your problem is.
But I have described here what I have done to build npp and scintilla with VS2017 community edition.
The steps for plugins should be quite similar.
A more detailed version include a lot of screenshots can be downloaded or viewed from here
If something is unclear, let us know. (together with the name of the plugin you tried to build)
Cheers
Claudia