Now theres something I do want to point out.
She was covered in soot and ash, her armor burnt in places. She stared at you blankly, her eyes unfocused and her sword held limply in her hand. The room almost immediately smelled of smoke and burnt wood, and you rose to check on her a moment later.
Its clear that Berserker tracked down, fought, and found 'Arthur' and then wherever they were fighting caught fire and left Berserker like this. However….I don't think it actually was Artoria, or proto-arthur, or MHX or hell any saberface. Theres a reason for this
The Berserker made no motion at your sudden approach, though her head tracked you as you briefly looked her over. Beyond the ever-present wound on her stomach however, and some faint cuts across the armor. She seemed uninjured.
Notice how the cuts are light. Unless Berserker had just begun her fight against 'Arthur' then don't you think she'd have more grevious wounds? I'm sure Arthur/Artoria would take a fight against Mordred quite seriously. Which is why I think I may know who she fought.
Think about it, the light cuts, soot and burnt pieces of armor….I think this was Jeanne D'arc Alter. It would explain the lack of damage on Berserker, as well as why wherever she was fighting was burning. "Now why would Berserker think this is Arthur" you must be asking. Simple answer. Remember how in Fate/Zero Gille De Rais had believed Artoria was Jeanne due to her appearance. Perhaps its a similar situation for Mordred, by virtue of being a psuedo-saberface Mordred had (due to madness enhancement) believed it to be Arthur…..or I'm looking too far into all this and her saying Arthur meant literally nothing.