In my previous playthrough I found that there's a way to avoid the fight with Llengrath (AKA THE ULTIMATE DRAGON BATTLE) if you say you don't want to fight and then pass a resolve or intellect threshold. But this time through I don't have that option.

The only real difference I can think of is that last time I had Concelhaut's phylactery as my pet, but this time I sold the phylactery to a random merchant.

So am I missing something or is Llengrath simply out to get me no matter what if I don't have the phylactery?

You could enable the option to see the attribute checks to verify, but yeah, its probably because you don't have the phylactery

I did enable that option.
And yes, once I loaded and returned with the skull I had the option of offering to give it to her and from there I had the option of reaching a peaceful solution.