Story related, can't be missed as long as you follow the main quest line.
If you're having trouble with him, my best advice is to use a hit and run approach. Dodge and roll a lot because he blocks often and hits hard, even if you're over leveled. When he starts teleporting around, mash that dodge button until he stops for some rest before you try hitting him. He also has a couple buffs and debuffs he can apply. He can hit you with a sort of ice bomb that slows your movements and causes Geralt to look a bit frosty. When you see that same kind of effect on Eredin, it functions as more armor, meaning your strikes do less damage. You can't do anything when he slows you besides wait until the debuff expires, but you can get rid of his ice armor by blasting him with Igni. He does teleport the both of you to a different location for part of the fight, but the same tactics apply regardless of where you're fighting him.
After the cutscene where he tells you his secret in his dying breath, the trophy should pop.