Q:

Easiest path for Microsoft Web API to generate WSDL

Web API, unlike some of the older Microsoft web service technologies, does not generate WSDL documentation automatically. 
What's the easiest way for it to automatically generate (or quickly manually generate) the WSDL associated with each service call? 

A:

Web API is technology to create REST services and REST services don't use WSDL so there is no way for your service to generate one.  WSDL 1.1 supported by MS and WCF even doesn't offer ability to describe REST services. 
WSDL 2.0 or WADL offers REST service description but MS and WCF supports neither of them and these standards were not broader accepted and supported yet. 