Simply said, the MethodDescriptor constructors have been changed in WebLogic and that specific one has been removed.
In wl12 the class is packaged in wlthint3client.jar, weblogic.server.merged.jar, com.bea.core.weblogic.rmi.client_188.8.131.52.jar , com.oracle.webservices.wls.jaxrpc-client.jar, com.oracle.webservices.wls.jaxws-wlswss-client.jar
You have only 2 solutions: either to use a wlfullclient.jar from the previous release of WL, or to regenerate the stub with WL12 appc.
see also http://stackoverflow.com/questions/25630220/oracle-iam-platform-utils-nosuchserviceexception-java-lang-reflect-invocationt/32314398#32314398 and https://liujinyao.wordpress.com/2015/05/11/java-lang-nosuchmethoderror-weblogic-rmi-internal-methoddescriptor-ljavalangreflectmethodljavalangclasszzzziiv/
As mentioned by Özkan Pakdil in the comment, it's much simpler to "delete *******_stub.class from client jar and deploy the ear that way so at the end wl12 generates it on the fly"