For PVP: Click on Duel/Colosseum.
Use "Tab" to select the one you want to fight. It should have a yellow frame now. Press Space/Enter and hold it until you are finished hitting.
Like this: http://www.pic-upload.de/view-19231843/DoTD1.jpg.html and hold enter/space
 "Aground"){kind=icon}
 "The Keeper Of 4 Elements"){kind=icon}
 "Dawn of the Dragons"){kind=icon}