You don’t.
You ‘fight’ Eizen once when you are getting Edna. and the other time after you have beaten Tiamat in Glavlind Basin. The reason there is a 2 though, is that you technically can beat him on the first encounter (he only has about 11K health then) but he uses the arte that all true dragons have access to – a self restoring arte. Tiamat has it as well, but due to Tiamat’s birth method, it feeds off malevolence. When [spoilers] happens, the malevolence isnt present on the battlefield any more, blocking Tiamat’s arte. Tiamat drops a orb which prevents Eizen from using the true form of the arte, allowing you to beat him.

http://gameswilo.com/tales-of-zestiria-enemies-locations-stats-and-drop-guide/