How do you fight grand master greevil or master greevil more than once

well you can do it the easy way or the hard way.
easy way: get action replay and get the code for all dungeons and go to the last mystifing forest you see . hard way : This MAY work. beat the whole dungeon again and it takes you to him : P.S. it is Grand Master of All Things Bad!

Don't listen to him. Just go to where Greevil is positioned which is at the top floor of Citadark Isle or the dome of the Cipher Headquarters where you fought Eldes, Ardos, Gorigan, and Snattle and you can battle Greevil again. He'll be waiting for you and all his Pokemon are at level 50.