Java Refactoring Strategy for Legacy Code

I have been asked to unit test some legacy code.

Currently, the code is tightly coupled with a 3rd party library both in terms of method calls and types used.

My first thought is to stub the 3rd party library (using an appropriate mocking framework) such that I can easily test the code of interest rather than the 3rd party library. However, to do this, I need to refactor some of the code so that the code of interest is isolated from the external library dependency.

My initial thought was to extract an interface and use a wrapper to make calls to the library. However, to entirely decouple the library I need to remove library specific types too, not just the method calls e.g.

LibrarySpecificType[] myVar = wrappedLibrary.DoX();

Although I have wrapped my library call in the above example, it still returns a library specific type, so it is still somewhat coupled.

How do I get around this? What is the best strategy to refactor the code to enable this?

Thanks!