In the Dragon Rescue the Endboss is the Transmutation Dragon, which sometimes deals multiple Damage killing up to 3 Dragons at once. Is this a Bug, because I don´t see that this Dragon has a special Skill ?

No it is as you said, very strong. None are able to kill them in one fight. It's best to save your strongest fighters for fights right before a door and the last boss.,and FYI the higher the Rarity, the stronger the opponents.

BTW closing this, there is a rescue topic in the Game events section, put all questions and concerns there