I told the dragon to help him, got the amulet to the dragon slayer and got this new dragon spell. Now the dragon is still friendly to me as long as I don't attack her. So I ran to her treasure-room and was infight. the Dragon will not run to you because he is blocked by the small creatures. So I spawned 2 Ogres with kana and kited the dragon with them until every minion was dead. After that I used Kana und kited the Dragon in a circle while he was singing his songs. I spawn this Ice attack that goes in 7 Directions every 5 Phrases. I did this for 20min and the Dragon died... my Party was counting the coins from the treasure during the kite session.
And thats how you kill a slow Dragon... running in a circle and singing Songs ^^