Now I'm not saying Zero is stupid... 
But he makes some pretty stupid choices sometimes...
*facepalm*
Anyway. Hope you don't mind the little fight picking Zarkun. Felt right and in character after I read your post. 
Also, maybe attacking an S-Class would get him grounded from the mission. I needed an excuse for someone other than him to go first and pick a fight with Hemlock. 
Maybe another S-Class Mage or two could join to help hurt the dragon. I need it before I can get rid of Hemlock XD
Or get rid of him during the next plot thing. Idk... Any suggestions?