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So, as it turns out, from Hell Bent,
Ashildr is the last surviving immortal being in the universe. It seems unlikely that she is the only Mire medical kit generated immortal, or is she instead the last? There was some uncertainty (in the Doctor's mind) as to how long the Medical kit would even work...
Is there any explanation for this?
There is no explanation for Ashildr's assertion that she is the only surviving immortal at that point. You saw the same episode the rest of us saw, and that's all we've got.
There is, in fact, no particular reason to believe her, given that she's demonstrated that she, like the Doctor, is not above deception.
Whether or not it's true is not isn't really relevant to the story. The story doesn't hinge on how many immortals are left, after all; only that she's one of them, and she's exactly where the Doctor expected her to be (and vice versa).
Purely speculatively, she didn't have a time machine, so was only moving forwards unless she got a ride with someone else. It might have been too close to the end for people who could move back in time to be comfortable.
The thing to remember is that Ashildr is NOT IMMORTAL. Seriously, all she has is a bit of advanced medical technology repairing her body. All the doctor did was wave his magic sonic wand at alien technology that already existed.
Its completely ridiculous to think that Ashildr went the long way around and survived for billions of years. Absolutely absurd. The only way the story makes sense for me, is that either she was working with the Time Lords who created that reality bubble for her and zapped her there from the past, in order to secure Clara Oswald's death, or that Ashildr herself went the long way around to the year 6000 or whatever until there was available time tech, used it to travel to that location figuring she would run into The Doctor.
I think the Time Lord theory is more probable given that Ashildr was keen on the situation from the jump, and had her memory as if they just last saw each other.
