I tried pythonnet with pypy on Win10. It builds fine, but then fails to import clr. The crash happens after this call in the screenshot.

Any tips/tools for debugging this?

pypy
Python 2.7.12 (aff251e54385, Nov 09 2016, 17:54:55)
[PyPy 5.6.0 with MSC v.1500 32 bit] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>> import clr
Attempting to load 'Python.Runtime' using standard binding rules.
'Python.Runtime' not found using standard binding rules.
Attempting to load Python.Runtime from: 'C:\Python\pypy2-v5.6.0-win32\site-packages\Python.Runtime.dll'.
Success loading 'Python.Runtime' from: 'C:\Python\pypy2-v5.6.0-win32\site-packages\Python.Runtime.dll'.
RPython traceback:
  File "pypy_module_cpyext_2.c", line 26114, in type_realize
  File "pypy_module_cpyext_2.c", line 31744, in _type_realize
Fatal RPython error: AssertionError

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

Here is the detailed thread about the issue