More answers are addressing how you got to this point. Many of them give some suggestions to remedy the situation, but I'd like to throw my answer in to give the short answer.
What to do when code reviews are "too hard?"
- Go back to the main line code branch
- Write tests for the functionality you've refactored (e.g. functional tests)
- Get the tests to pass
- Merge the tests into the code that's "hard to test"
- Do the tests still pass?
Yes
You developers were great! Cats back for everyone!
(or for those who didn't grow up watching "The Simpsons" on U.S. television: If the tests are passing, skip trying to look at the differences and have the developer lead you on a tour of the changes)
No
Keep refactoring and adding test coverage until the tests are passing.