How can I secure a solo-authored paper as a PhD student, while working with a supervisor who has reputation of forcing coauthorship?
Does your superviser deserve an authorship? Check the following:


Have they contributed to the research design, in form of suggesting a topic to investigate, advising on methodology, suggesting literature?
Were they involved in evaluation of results, in form of checking them and contributing an opinion on whether the work is going in the right direction?
Have they contributed to writing a paper, in form of contributing an introduction or abstract, or editing the manuscript?
(*) Have you used any of the facilities or financial streams (e.g. stipendship) funded from your supervisor's research budget?


If any of the above is true, then your superviser may actually deserve an authorship.  You can also check Nature Authorship Guidelines or a similar document in your university.

If you answered no to all of the above, then it is absolutely appropriate for you to publish on your own. Simply use the Authorship Guidelines to politely explain to your supervisor why you believe that they should not be included as an author. You may want to discuss your position with a dedicated unit in your University/Department, such as Research Office or the Head of the Department.

UPD: (*) As many people commented (and I agree), criteria (4) is controversial. In many areas providing funding on its own does not qualify for an authorship (see e.g. Vancouver protocol). However, in many departments it is a norm to always include the Head or Director of the Lab as the last author, and it may not be easy to challenge this agreement for a PhD student, particularly if you benefit from this funding.